Optical wavelength multiplex transmission method and optical dispersion compensation method

ABSTRACT

The invention provides an optical wavelength multiplex transmission method wherein a band in the proximity of a zero dispersion wavelength of an optical fiber is used and optical signals are disposed at efficient channel spacings taking an influence of the band, the wavelength dispersion and the four wave mixing into consideration to realize an optical communication system of an increased capacity which is not influenced by crosstalk by FWM. When optical signals of a plurality of channels having different wavelengths are to be multiplexed and transmitted using an optical fiber, a four wave mixing suppressing guard band of a predetermined bandwidth including the zero-dispersion wavelength λ 0  of the optical fiber is set, and signal light waves of the plurality of channels to be multiplexed are arranged on one of the shorter wavelength side and the longer wavelength side outside the guard band.

This application is a divisional of application Ser No. 08/917,292,filed Aug. 25, 1997, now ALLOWED, which was a continuation ofapplication Ser. No. 08/781,137, filed Jan. 9, 1997, now U.S. Pat. No.5,696,614 which was a continuation of application Ser. No. 08/233,830filed Apr. 26, 1994, now ABANDONED.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical wavelength multiplex transmissionmethod which uses a band around a zero dispersion wavelength of anoptical fiber and an optical dispersion compensation method forcompensating for waveform degradation by a synergetic effect(hereinafter referred to as SPM-GVD effect) of self phase modulation(SPM) and chromatic dispersion (group velocity dispersion: GVD) which isone of several restrictive factors to the transmission distance and thetransmission rate in a long-haul, very high-speed optical communicationsystem which employs, for example, an erbium-doped optical fiberamplifier (Erbium-Doped Fiber Amplifier, hereinafter referred to asEDFA).

2. Description of the Related Art

In response to a remarkable increase in the amount of information inrecent years, a communication system of a large capacity has becomerequired, and investigations for construction of large capacitycommunication systems are frequently performed.

For realization of a large capacity communication system, realization byan optical communication system is considered most promising. Atpresent, an optical amplifier multi-repeater system which employs EDFAsis being put into practical use together with, for example, a 2.4 Gb/soptical communication system. In the future, it is forecast that theamount of information will increase progressively as theinformation-oriented trend advances. It is therefore demanded to buildup an optical communication system of an increased capacitycorresponding to such increase of the amount of information.

Various methods are available to increase the capacity of an opticalcommunication system, including a TDM (time-division multiplexing)method which involves multiplexing on the time base in order to increasethe transmission rate, and a WDM (wavelength-division multiplexing:wherein the wavelength spacing is comparatively great and is called WDM,and wavelength-division multiplexing which involves high concentrationmultiplexing and is called FDM (frequency-division multiplexing)) methodwhich involves multiplexing on the optical wavelength base.

Of the available methods, a multiplexing method like the TDM methodrequires an increase of speed of operation of electronic circuits in atransmitter and a receiver in order to increase the transmission rate.At present, several tens Gb/s is considered to be the limit to the speedof operation.

In contrast, with the WDM (FDM) method which makes use of the wide bandproperty of an optical fiber, an increase of capacity to several tens toseveral hundreds Gb/s is possible by simultaneous exploitation of anincrease of the transmission rate, and also the burden to electroniccircuits is reduced since multiplexing and demultiplexing are performedsimply in an optical region by means of an optical multiplexingapparatus and an optical demultiplexing apparatus (MUX/DEMUX) whichemploy optical couplers, optical filters and like elements.

In the WDM (FDM) method which involves wavelength multiplexing on theoptical frequency base, however, an available band is restricted by gainband dependency of an optical amplifier or wavelength dependency of anoptical part. Accordingly, in order to achieve an increase in capacityby multiplexing, the channel spacing must necessarily be decreased todecrease the bandwidth indicated by all channels. Further, in opticaltransmission of multi-Gigabits, the wavelength of an optical signal mustnecessarily be set in the proximity of a zero dispersion wavelength ofan optical fiber since, otherwise, waveform degradation is caused bychromatic dispersion of the optical fiber.

In an optical communication system to which the WDM (FDM) method isapplied in order to achieve such an increase in capacity as describedabove, however, if the channel spacing is decreased (taking thebandwidth into consideration) and optical signals are set in theproximity of a zero dispersion wavelength of the optical fiber (takingthe chromatic dispersion into consideration), an influence of anon-linear effect of the optical fiber, particularly of four wave mixing(hereinafter referred to as FWM), becomes significant, and there is asubject to be solved in that the transmission may be disabled bycrosstalk from another channel by such FWM. A similar subject resides inanother case wherein wavelength multiplex transmission must be performedin a band in the proximity of the zero dispersion wavelength in order toachieve, for example, upgrading of an existing transmission line.

Meanwhile, as a factor of degradation of the transmission characteristicin the optical amplifier multi-repeater WDM method which particularlymakes use of a band in the proximity of a zero dispersion wavelength ofan optical fiber, crosstalk by FWM mentioned above is pointed out. Theoccurrence efficiency of such FWM depends upon the relationship betweenthe zero dispersion wavelength of the optical fiber transmission lineand the arrangement of channels.

Three characteristics including: 1. a zero dispersion wavelength, 2. adeviation in zero dispersion wavelength and 3. a dispersion slope(second-order dispersion) are listed as required characteristics for anoptical fiber in the WDM method. Those characteristics are closelyrelated to five factors including: a. wavelength multiplexing signalbandwidth, b. gain bandwidth of the EDFA among various opticalamplifiers, c. guard band for suppressing FWM (to which the presentinvention is directed), d. limitation bandwidth by an SPM-GVD effect,and e. presence or absence of an inserted optical dispersioncompensator.

By the way, as factors which restrict an increase in distance and anincrease in speed of an optical communication system, there arelimitations of the loss by an optical fiber and bandwidth limitation bychromatic dispersion. The loss limitation has been almost solved byrealization of EDFAs, and it is possible to build up a very long-hauloptical communication system for several thousand km or more.

However, the repeater span in a multi-repeater optical amplificationsystem is restricted principally by two factors including: 1. opticalSNR (signal to noise ratio) degradation caused by accumulation of ASE(spontaneous emission) from optical amplifier-repeaters, and 2. waveformdegradation by an SPM-GVD effect caused by a Kerr effect.

It is already known that, of the two factors, the waveform degradationby an SPD-GVD effect can be compensated for using an optical dispersioncompensator having a dispersion value of the opposite positive ornegative sign to that of the optical fiber transmission line, and thewaveform degradation by an SPM-GVD effect and a dispersion compensationeffect can be simulated readily by solving a non-linear Schroedingerequation using the split-step Fourier method.

An optical dispersion compensator used for the object described above isrequired to cope with a dispersion amount of an optical fiber of acorresponding repeater section and to allow reduction of the number ofsteps and of the time necessary to realize an optimum dispersioncompensation amount and reduction of the cost. Further, the opticaldispersion compensation technique is important not only for a 1.55 μmdispersion shifted fiber (hereinafter referred to as DSF) transmissionline network being laid at present but also for a long-haul, veryhigh-speed optical communication system and an optical communicationsystem of the WDM (FDM) method which make use of an existing 1.3 μm zerodispersion single mode fiber (hereinafter referred to as SMF)transmission line network.

In a very long-haul optical communication system for several thousand kmor more, it is considered desirable to use the zero dispersionwavelength λ₀ of the optical fiber transmission line in order to preventthe dispersion penalty and to use the ordinary dispersion region(dispersion value D<0) of the optical fiber in order to minimize thenon-linear effect. In order to satisfy the two contradictoryrequirements, a countermeasure has been proposed which makes use of theordinary dispersion region for the transmission line and employs anoptical dispersion compensator to reduce the apparent dispersion valueequal to zero. The optical dispersion compensation technique iseffective not only for DSF transmission but also for SMF transmissionhaving a high dispersion value of approximately 18 ps/nm/km.

Various types of optical dispersion compensators have been proposedincluding dispersion compensating fiber type optical dispersioncompensators, transversal filter type optical dispersion compensatorsand optical resonator type optical dispersion compensators. At present,a dispersion compensating fiber is considered promising from itsadvantage in that the dispersion compensation amount can be adjustedreadily by varying the length of the fiber, and dispersion values higherthan −100 ps/(nm·km) have been obtained by contriving the profile of thecore.

The zero dispersion wavelength of an actual optical fiber transmissionline presents a deviation in a longitudinal direction. Further, in anoptical communication system on land, since it is difficult to set therepeater span to a fixed value (as in a submarine optical communicationsystem), the dispersion amount is not always fixed among differentrepeater sections. Therefore, ideally an optical dispersion compensatorhaving an optimum dispersion compensation amount is inserted into eachrepeater section after an actual dispersion amount is measured for therepeater section. However, there is a subject in that such operationrequires a great number of steps of operation, long time and a high costto realize optimum optical dispersion compensators including measurementof dispersion amounts.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an opticalwavelength multiplex transmission method wherein, where a band in theproximity of a zero dispersion wavelength of an optical fiber is used,optical signals are disposed at efficient channel spacings taking aninfluence of the band, the chromatic dispersion, and the FWM intoconsideration, to realize an optical communication system of anincreased capacity which is not influenced by crosstalk by FWM.

It is another object of the present invention to provide an opticalwavelength multiplex transmission method wherein the relationshipbetween characteristics required for an optical fiber, particularly, thezero-dispersion wavelength and the deviation in zero-dispersionwavelength, and five specific factors related to the characteristics ismade clear to allow establishment of channel arrangement of andtransmission line designing for signal light by an optical amplifiermulti-repeater WDM method.

It is a further object of the present invention to provide an opticaldispersion compensation method by which waveform degradation by anSPM-GVD effect can be compensated for readily without designing orproducing optical dispersion compensators suitable for individualtransmission lines and dispersion compensation can be performedeffectively even when the optical power is not so high that SPM (selfphase modulation) does not take place very much, but only waveformdegradation is caused by chromatic dispersion (GVD), thereby to reducethe number of steps and the time required to build up an opticalcommunication system and to achieve reduction of the cost.

In order to attain the objects described above, according to an aspectof the present invention, there is provided an optical wavelengthmultiplex transmission method for multiplexing signal light waves of aplurality of channels having different wavelengths and transmitting themultiplexed signal light using an optical fiber, wherein a four wavemixing suppressing guard band of a predetermined bandwidth including azero-dispersion wavelength of the optical fiber is set, and the signallight waves of the plurality of channels to be multiplexed are arrangedon one of a shorter wavelength side and a longer wavelength side outsidethe guard band.

In the optical wavelength multiplex transmission method, when signallight waves of a plurality of channels having different wavelengths aremultiplexed and transmitted using an optical fiber, since the signallight waves of the plurality of channels to be multiplexed are arrangedon one of the shorter wavelength side and the longer wavelength sideoutside the four wave mixing suppressing guard band of the predeterminedbandwidth including the zero-dispersion wavelength of the optical fiber,possible four wave mixing is suppressed. Consequently, an influence fromanother channel by crosstalk is suppressed.

According to another aspect of the present invention, there is providedan optical wavelength multiplex transmission method for multiplexingsignal light waves of a plurality of channels having differentwavelengths and transmitting the multiplexed signal light using anoptical fiber, wherein a four wave mixing suppressing guard band of apredetermined bandwidth including a zero-dispersion wavelength of theoptical fiber is set, and the signal light waves of the plurality ofchannels to be multiplexed are arranged on the opposite sides of ashorter wavelength side and a longer wavelength side outside the guardband.

In the optical wavelength multiplex transmission method, since signallight waves of a plurality of channels to be multiplexed are arranged onthe opposite sides of the shorter wavelength side and the longerwavelength side outside the four wave mixing suppressing guard band,possible four wave mixing is suppressed. Consequently, an influence fromanother channel by crosstalk is suppressed and efficient utilization ofthe band can be achieved simultaneously.

The bandwidths of the guard bands may be set in an asymmetricalrelationship on the shorter wavelength side and the longer wavelengthside with respect to the zero-dispersion wavelength of the opticalfiber. In this instance, the channel spacings between adjacent ones ofthe signal light waves of the plurality of channels may be set differenton the shorter wavelength side and the longer wavelength side outsidethe guard band. Due to the channel spacings thus set, four wave mixinglight produced between a signal light wave on the shorter wavelengthside and another signal light wave on the longer wavelength side isprevented from coinciding with any of the wavelengths of the signallight waves.

Alternatively, the channel spacings between adjacent ones of the signallight waves of the plurality of channels on each of the shorterwavelength side and the longer wavelength side outside the guard bandmay be set to an integral number times a constant. Due to the channelspacings thus set, in addition to the advantage that an influence fromanother channel by crosstalk is suppressed, the channels on the shorterwavelength side and the longer wavelength side outside the guard bandcan be controlled using Fabry-Perot interferometers of a samecharacteristic. In this instance, preferably the channel spacingsbetween the channels of the signal light waves of the plurality ofchannels on the opposite sides of the guard band are set to the integralnumber times the constant. Due to the channel spacings thus set, thechannels on the opposite sides of the shorter wavelength side and thelonger wavelength side outside the guard band can be controlledsimultaneously using a single Fabry-Perot interferometer of a samecharacteristic. Or else, the signal light waves of the channels may bearranged such that the signal light waves of no pair or only one pair ofones of the channels have dispersion values which have an equal absolutevalue. The arrangement further suppresses four wave mixing so that aninfluence from another channel by crosstalk can be further suppressed.

With the optical wavelength multiplex transmission methods describedabove, the following effects or advantages can be anticipated.

First, an influence of four wave mixing can be suppressed and the bandcan be utilized efficiently by arranging signal light waves efficiently.An optical communication system of a large capacity can be realizedwhile maintaining high transmission quality.

Second, even when a zero-dispersion wavelength is positioned within aband of an optical amplifier or within a band of an optical part, signallight waves can be arranged efficiently and compactly while suppressingan effect of four wave mixing within the limited band.

Third, since the channel spacings on the transmission side can becontrolled by way of a single or a pair of Fabry-Perot interferometersand an interferometer of the same characteristic to that of theinterferometers on the transmission side can be used also on thereception side, control on the transmission side can be simplified andselective reception is facilitated.

According to a further aspect of the present invention, there isprovided an optical wavelength multiplex transmission method formultiplexing signal light waves of a plurality of channels havingdifferent wavelengths and transmitting the multiplexed signal lightusing an optical fiber, wherein, taking a zero-dispersion wavelength λ₀of the optical fiber and a zero-dispersion wavelength deviation range±Δλ₀ of the optical fiber in its longitudinal direction intoconsideration, the signal light waves of the plurality of channels to bemultiplexed are arranged on a shorter wavelength side than a shorterwavelength end λ₀−Δλ₀ of the zero-dispersion wavelength deviation rangeof the optical fiber.

In the optical wavelength multiplex transmission method, when signallight waves of a plurality of channels having different wavelengths aremultiplexed and transmitted using an optical fiber, since the signallight waves of the plurality of channels to be multiplexed are arrangedon the shorter wavelength side than the shorter wavelength end λ₀−Δλ₀ ofthe zero-dispersion wavelength deviation range of the optical fiber, thezero-dispersion wavelength deviation in the longitudinal direction ofthe optical fiber is taken into consideration and controlled on theshorter wavelength side of the zero-dispersion wavelength.

A four wave mixing suppressing guard band Δλ_(g) may be provided on theshorter wavelength side than the shorter wavelength end λ₀−Δλ₀ of thezero-dispersion wavelength deviation range of the optical fiber, and thesignal light waves of the plurality of channels may be arranged on ashorter wavelength side than a wavelength λ₀−Δλ₀−Δλ_(g). In thisinstance, since the signal light wave of the plurality of channels arearranged on the shorter wavelength side than the wavelengthλ₀−Δλ₀−Δλ_(g), taking the four wave mixing suppressing guard band Δλ_(g)into consideration, the zero-dispersion wavelength deviation in thelongitudinal direction of the optical fiber is taken into considerationand controlled on the shorter wavelength side of the zero-dispersionwavelength. Thus simultaneously, an influence from another channel bycrosstalk is suppressed.

According to a still further aspect of the present invention, there isprovided an optical wavelength multiplex transmission method formultiplexing signal light waves of a plurality of channels havingdifferent wavelengths and transmitting the multiplexed signal lightusing an optical fiber, wherein, taking a zero-dispersion wavelength λ₀of the optical fiber and a zero-dispersion wavelength deviation range±Δλ₀ of the optical fiber in its longitudinal direction intoconsideration, the signal light waves of the plurality of channels to bemultiplexed are arranged on a longer wavelength side than a longerwavelength end λ₀+Δλ₀ of the zero-dispersion wavelength deviation rangeof the optical fiber.

In the optical wavelength multiplex transmission method, when signallight waves of a plurality of channels having different wavelengths aremultiplexed and transmitted using an optical fiber, since the signallight waves of the plurality of channels to be multiplexed are arrangedon the longer wavelength side than the longer wavelength end λ₀+Δλ₀ ofthe zero-dispersion wavelength deviation range of the optical fiber, thezero-dispersion wavelength deviation in the longitudinal direction ofthe optical fiber is taken into consideration and controlled on thelonger wavelength side of the zero-dispersion wavelength.

A four wave mixing suppressing guard band Δλ_(g) may be provided on thelonger wavelength side than the longer wavelength end λ₀+Δλ₀ of thezero-dispersion wavelength deviation range of the optical fiber, and thesignal light waves of the plurality of channels may be arranged on alonger wavelength side than a wavelength λ₀+Δλ₀+Δλ_(g). Due to theprovision of the four wave mixing suppressing guard band Δλ_(g) and thearrangement of the signal light waves, the zero-dispersion wavelengthdeviation in the longitudinal direction of the optical fiber is takeninto consideration and controlled on the longer wavelength side of thezero-dispersion wavelength, and simultaneously, an influence of anotherchannel by crosstalk is suppressed.

The signal light waves of the plurality of channels may be arrangedwithin a transmissible band defined by an allowable dispersion valuedetermined from a synergetic effect of self phase modulation and groupvelocity dispersion in the optical fiber. Where the signal light wavesare arranged in this manner, they can be arranged taking wavelengthdegradation by an SPM-GVD effect into consideration. Further, althoughSPM does not take place very much and only waveform degradation bychromatic dispersion (GVD) occurs when the optical power is not veryhigh, the signal light arrangement can be performed also taking suchwaveform degradation into consideration.

The signal light waves of the plurality of channels may be arrangedoutside the transmissible band defined by the allowable dispersion valuedetermined from the synergetic effect of self phase modulation and groupvelocity dispersion in the optical fiber, and the zero dispersionwavelength λ₀ of the optical fiber may be apparently shifted using anoptical dispersion compensator to apparently arrange the signal lightwaves of the plurality of channels into the transmissible band. Due tothe arrangement of the signal light waves and the shift of the zerodispersion wavelength λ₀, the signal light waves can be arranged takingwaveform degradation by an SPM-GVD effect into consideration.

The optical wavelength multiplex transmission method may be constructedsuch that, taking a dispersion compensation amount deviation range±δλ_(DC) of the optical dispersion compensator into consideration, aband Δλ_(WDM) within which the signal light waves of the plurality ofchannels are to be arranged is set expanding the same by the dispersioncompensation amount deviation range δλ_(DC) on the opposite sides of thelonger wavelength side and the shorter wavelength side. Due to the bandΔλ_(WDM) thus set, the signal light waves can be arranged taking thedispersion compensation amount deviation of the optical dispersioncompensator into consideration.

The signal light waves of the plurality of channels may be arranged in again band of an optical amplifier connected to the optical fiber. Due tothe arrangement of the signal light waves, the powers of the signallight waves can be made equal to each other and also the receivecharacteristics of the signal light waves can be made equal to eachother.

A band Δλ_(WDM) within which the signal light waves of the plurality ofchannels are to be arranged may be set expanding the same in accordancewith optical wavelength variations of the signal light waves of theplurality of channels. Due to the band Δλ_(WDM) thus set, theproductivity of light sources of the signal light waves and thevariation of each signal light wave by the wavelength control accuracyare taken into consideration.

With the optical wavelength multiplex transmission methods describedabove, the following effects or advantages can be anticipated.

First, in a wavelength division multiplexing method which makes use of aband in the proximity of the zero-dispersion wavelength λ₀ of theoptical fiber, the signal light waves of the individual channels can bearranged without being influenced by four wave mixing, andsimultaneously, required characteristics regarding the zero-dispersionwavelength λ₀ for an optical fiber transmission line to be laid can bemade clear. Consequently, channel arrangement of and transmission linedesigning for signal light by an optical amplifier multi-repeater WDMmethod can be established.

Second, the zero-dispersion wavelength deviation in the longitudinaldirection of the optical fiber is taken into consideration andcontrolled, and simultaneously, an influence of four wave mixing issuppressed so that an influence from another channel by crosstalk issuppressed. Consequently, a high degree of transmission accuracy can bemaintained.

Third, signal light waves can be arranged taking waveform degradation byan SPM-GVD effect into consideration, and where the signal light wavesof different channels are arranged in the gain bandwidth Δλ_(EDFA) ofthe EDFA, the powers of the signal light waves can be made equal to eachother and the receive characteristics of the signal light waves can bemade equal to each other.

Fourth, where a signal light band is set expanding the same inaccordance with optical wavelength variations of the signal light wavesof the channels, the variations of the signal light waves arising fromthe productivity and/or the wavelength control accuracy of light sourcesof the signal light waves such as semiconductor lasers are taken intoconsideration, and where an optical dispersion compensator is employed,by setting the signal light band expanding the same by a dispersioncompensation amount deviation range on the opposite sides of the shorterwavelength side and the longer wavelength side, also the dispersioncompensation amount deviation of the optical dispersion compensator istaken into consideration. Consequently, optical transmission of higherreliability can be achieved.

According to a yet further aspect of the present invention, there isprovided an optical dispersion compensation method for compensating fora dispersion amount of an optical transmission system which includes atransmitter, a repeater and a receiver and transmits signal light fromthe transmitter to the receiver by way of the repeater, comprising thesteps of preparing in advance two kinds of optical dispersioncompensator units having dispersion amounts having different positiveand negative signs, inserting the two kinds of optical dispersioncompensator units separately into the optical transmission system, andselecting one of the two kinds of optical dispersion compensator unitswhich provides a better transmission characteristic to the opticaltransmission system and incorporating the selected optical dispersioncompensator unit into the optical transmission system.

In the optical dispersion compensation method, since two kinds ofoptical dispersion compensator units having dispersion amounts havingdifferent positive and negative signs are prepared in advance andinserted separately into an optical transmission system to select one ofthe two kinds of optical dispersion compensator units which provides abetter transmission characteristic to the optical transmission system,the dispersion amount of the optical transmission system can becompensated for simply when an accurate dispersion amount cannot bemeasured but the zero-dispersion wavelength deviation can be grasped tosome degree.

According to a yet further aspect of the present invention, there isprovided an optical dispersion compensation method for compensating fora dispersion amount of an optical transmission system which includes atransmitter, a repeater and a receiver and transmits signal light fromthe transmitter to the receiver by way of the repeater, comprising thesteps of preparing in advance two kinds of optical dispersioncompensator units having dispersion amounts having different positiveand negative signs, measuring a dispersion amount of the opticaltransmission system, and selecting one of the two kinds of opticaldispersion compensator units which has a dispersion amount whose sign isopposite to that of a measured dispersion amount and incorporating theselected optical dispersion compensator unit into the opticaltransmission system.

In the optical dispersion compensation method, since two kinds ofoptical dispersion compensator units having dispersion amounts havingdifferent positive and negative signs are prepared in advance and, whenthe dispersion amount of an optical transmission system can be measured,the dispersion amount is measured and then one of the two kinds ofoptical dispersion compensator units which has a dispersion value whosesign is opposite to that of a thus measured dispersion value isselected, the dispersion amount of the optical transmission system canbe compensated for further reliably.

According to a yet further aspect of the present invention, there isprovided an optical dispersion compensation method for compensating fora dispersion amount of an optical transmission system which includes atransmitter, a repeater and a receiver and transmits signal light fromthe transmitter to the receiver by way of the repeater, comprising thesteps of preparing in advance a plurality of kinds of optical dispersioncompensator units having different dispersion amounts having differentpositive and negative signs, selectively inserting the plurality ofkinds of optical dispersion compensator units into the opticaltransmission system changing the installation number and the combinationof the optical dispersion compensator units, and selecting aninstallation number and a combination of the optical dispersioncompensator units from within the plurality of kinds of opticaldispersion compensator units which provide a good transmissioncharacteristic to the optical transmission system and incorporating theoptical dispersion compensator units of the selected installation numberand combination into the optical transmission system.

In the optical dispersion compensation method, since a plurality ofkinds of optical dispersion compensator units having differentdispersion amounts having different positive and negative signs areprepared in advance and selectively inserted into an opticaltransmission system changing the installation number and the combinationof the optical dispersion compensator units and then an installationnumber and a combination of the optical dispersion compensator unitswhich provide a good transmission characteristic to the opticaltransmission system are selected from within the plurality of kinds ofoptical dispersion compensator units, the dispersion amount of theoptical transmission system can be compensated for simply and optimallywhen the zero-dispersion wavelength deviation is unknown or thezero-dispersion wavelength and the wavelengths of the signal light wavesare displaced by great amounts from each other.

According to a yet further aspect of the present invention, there isprovided an optical dispersion compensation method for compensating fora dispersion amount of an optical transmission system which includes atransmitter, a repeater and a receiver and transmits signal light fromthe transmitter to the receiver by way of the repeater, comprising thesteps of preparing in advance a plurality of kinds of optical dispersioncompensator units having different dispersion amounts having differentpositive and negative signs, measuring a dispersion amount of theoptical transmission system, and selecting an installation number and acombination of the optical dispersion compensator units from within theplurality of kinds of optical dispersion compensator units, with whichdispersion values of the signal light waves fall within a transmissibledispersion value range, in accordance with a measured dispersion valueand incorporating the optical dispersion compensator units of theselected installation number and combination into the opticaltransmission system.

In the optical dispersion compensation method, since a plurality ofkinds of optical dispersion compensator units having differentdispersion amounts having different positive and negative signs areprepared in advance and, when the dispersion amount of an opticaltransmission system can be measured, the dispersion amount is measuredand then an optimum installation number and an optimum combination ofsuch optical dispersion compensator units are selected in accordancewith a thus measured dispersion amount, the dispersion amount of theoptical transmission system can be compensated for so that it may fallwithin an allowable dispersion value range with certainty.

The optical dispersion compensator units may be additionallyincorporated into at least one of the transmitter, the repeater and thereceiver of the optical transmission system to incorporate the opticaldispersion compensator units into the optical transmission system.

When the optical transmission system performs optical wavelengthmultiplex transmission to multiplex and transmit signal light waves of aplurality of channels having different wavelengths, the signal lightwaves may be demultiplexed for each one wave by wavelengthdemultiplexing and the optical dispersion compensator units may beprovided for the individual channels of the signal light waves of thewavelengths in the optical transmission system, or the signal lightwaves may be demultiplexed for each plurality of waves and the opticaldispersion compensator units may be provided for the individual channelgroups each including a plurality of signal light waves in the opticaltransmission system, or else the optical dispersion compensator unitsmay be provided for all of the signal light waves of the plurality ofchannels in the optical transmission system.

Each of the optical dispersion compensator units may be additionallyprovided with an optical amplifier for compensating for an optical lossof the optical dispersion compensator unit. Due to the additionalprovision of the optical amplifier, the optical loss of each opticaldispersion compensator unit can be compensated for. In this instance, apair of optical amplifiers may be additionally provided at a precedingstage and a next stage to each of the optical dispersion compensatorunits. Due to the additional provision of the optical amplifiers, thenoise figure (hereinafter referred to as simply NF) of the opticalamplifier at the preceding stage can be set low.

The optical dispersion compensator units may be constructed as a packagewherein they are mounted on a circuit board so that the opticaldispersion compensator units may be replaced or incorporated in units ofa package. Due to the construction of the optical dispersion compensatorunits, the dispersion compensation amount can be varied readily.

According to a yet further aspect of the present invention, there isprovided an optical dispersion compensation method for compensating fora dispersion amount of an optical transmission system which includes atransmitter, a repeater and a receiver and transmits signal light fromthe transmitter to the receiver by way of the repeater, comprising thesteps of incorporating, in advance into at least one of the transmitter,the repeater and the receiver of the optical transmission system, aplurality of kinds of optical dispersion compensator units havingdifferent dispersion amounts having different positive and negativesigns in such a connected condition as to allow switching of a selectivecombination of the optical dispersion compensator units by means ofswitching means, and operating the switching means to select a suitablecombination of the optical dispersion compensator units from within theplurality of types of optical dispersion compensator units andincorporating the optical dispersion compensator units of the selectedcombination into the optical transmission system.

In the optical dispersion compensation method, since a plurality ofkinds of optical dispersion compensator units having differentdispersion amounts having different positive and negative signs areincorporated in advance in at least one of a transmitter, a repeater anda receiver of an optical transmission system in such a connectedcondition as to allow switching of a selective combination of theoptical dispersion compensator units by means of switching means, asuitable combination of the optical dispersion compensator units can beselected from within the plurality of types of optical dispersioncompensator units.

The switching means may be operated in response to a control signal fromthe outside. In this instance, the optical dispersion compensationmethod may be constructed such that the switching means is operated inresponse to a control signal from the receiver to switch the combinationof the optical dispersion compensator units while a transmissioncharacteristic of the optical transmission system is measuredsimultaneously by the receiver to determine a combination of the opticaldispersion compensator units which provides an optimum transmissioncharacteristic to the optical transmission system, and the switchingmeans is operated in response to another control signal from thereceiver to switch the combination of the optical dispersion compensatorunits to the determined combination which provides the optimumtransmission characteristic to the optical transmission system. Theswitching means may include a mechanical switch or an optical switch.

With the optical dispersion compensation methods described above, thefollowing effect or advantage can be achieved. In particular, waveformdeterioration by an SPM-GVD effect and/or the dispersion amount of aguard band can be compensated for readily without designing or producingoptical dispersion compensators suitable for individual transmissionlines, and reduction of the number of steps and the time required tobuild up an optical communication system can be realized.

Further objects, features and advantages of the present invention willbecome apparent from the following detailed description when read inconjunction with the accompanying drawings in which like parts orelements are denoted by like is reference characters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view illustrating the arrangement of signallight waves of a plurality of channels according to an opticalwavelength multiplex transmission method of a first preferred embodimentof the present invention;

FIG. 2 is a block diagram showing the construction of an optical WDMdistribution transmission system to which the optical wavelengthmultiplex transmission method of the first embodiment of the presentinvention is applied;

FIGS. 3 and 4 are diagrams illustrating operation of the firstembodiment of the present invention;

FIG. 5 is a diagrammatic view illustrating the arrangement of signallight waves of a plurality of channels according to an opticalwavelength multiplex transmission method of a second preferredembodiment of the present invention;

FIG. 6 is a similar view but illustrating the arrangement of signallight waves of a plurality of channels according to an opticalwavelength multiplex transmission method of a third preferred embodimentof the present invention;

FIG. 7 is a diagram illustrating operation of the third embodiment ofthe present invention;

FIG. 8 is a diagrammatic view illustrating the arrangement of signallight waves of a plurality of channels according to an opticalwavelength multiplex transmission method of a fourth preferredembodiment of the present invention;

FIG. 9 is a similar view but illustrating the arrangement of signallight waves of a plurality of channels according to an opticalwavelength multiplex transmission method of a fifth preferred embodimentof the present invention;

FIG. 10 is a similar view but illustrating the arrangement of signallight waves of a plurality of channels according to an opticalwavelength multiplex transmission method of a sixth preferred embodimentof the present invention;

FIGS. 11 and 12 are diagrams illustrating operation of the sixthembodiment of the present invention;

FIG. 13 is a diagrammatic view illustrating the arrangement of signallight waves of a plurality of channels according to an opticalwavelength multiplex transmission method of a seventh preferredembodiment of the present invention;

FIG. 14 is a similar view but illustrating a modification to thearrangement of signal light waves illustrated in FIG. 13;

FIG. 15 is a block diagram showing the construction of a regenerativerepeater system to which the optical wavelength multiplex transmissionmethod of the seventh embodiment of the present invention is applied;

FIG. 16 is a graph showing an ASE spectrum of a gain distribution of anEDFA after connection of EDFAs at four stages and illustrating a gainband of the EDFA;

FIG. 17 is a diagram illustrating the arrangement of an FWM suppressingguard band and channels;

FIG. 18 is a graph illustrating the relationship between a dispersionvalue of the channel 1 and crosstalk;

FIG. 19 is a graph illustrating the relationship between the opticalfiber input power and the regenerative repeater span;

FIG. 20 is a graph illustrating the signal light wavelength dependencyof the FWM occurrence efficiency;

FIG. 21 is a graph illustrating the relationship between the channelspacing and a guard band;

FIG. 22 is a graph illustrating the relationship of the zero-dispersionwavelength and the dispersion compensation amount to the zero-dispersionwavelength deviation in the seventh embodiment of the present invention;

FIG. 23 is a diagram illustrating the arrangement of signal light wavesof a plurality of channels according to an optical wavelength multiplextransmission method of an eighth preferred embodiment of the presentinvention;

FIG. 24 is a similar view but illustrating a modification to thearrangement of signal light waves illustrated in FIG. 23;

FIG. 25 is a graph illustrating the relationship of the zero-dispersionwavelength and the dispersion compensation amount to the zero-dispersionwavelength deviation in the eighth embodiment of the present invention;

FIG. 26 is a block diagram showing an optical dispersion compensationsystem to which an optical dispersion compensation method of a ninthpreferred embodiment of the present invention is applied;

FIG. 27 is a block diagram showing an optical dispersion compensationsystem to which an optical dispersion compensation method of a tenthpreferred embodiment of the present invention is applied;

FIG. 28 is a block diagram showing an optical dispersion compensationsystem to which an optical dispersion compensation method of an eleventhpreferred embodiment of the present invention is applied;

FIG. 29 is a block diagram showing a modification to the opticaldispersion compensation system shown in FIG. 28;

FIG. 30 is a block diagram showing another modification to the opticaldispersion compensation system shown in FIG. 28;

FIG. 31 is a block diagram showing an optical dispersion compensationsystem to which an optical dispersion compensation method of a twelfthpreferred embodiment of the present invention is applied;

FIG. 32 is a block diagram showing a modification to the opticaldispersion compensation system shown in FIG. 31;

FIG. 33 is a block diagram showing another modification to the opticaldispersion compensation system shown in FIG. 31;

FIG. 34 is a block diagram showing an optical dispersion compensationsystem to which an optical dispersion compensation method of athirteenth preferred embodiment of the present invention is applied;

FIG. 35 is a block diagram showing a modification to the opticaldispersion compensation system shown in FIG. 34;

FIG. 36 is a block diagram showing another modification to the opticaldispersion compensation system shown in FIG. 34;

FIG. 37 is a block diagram showing an optical dispersion compensationsystem to which an optical dispersion compensation method of afourteenth preferred embodiment of the present invention is applied;

FIGS. 38(a) and 38(b) are block diagrams showing a modification to theoptical dispersion compensation system shown in FIG. 37;

FIG. 39 is a block diagram showing another modification to the opticaldispersion compensation system shown in FIG. 37;

FIG. 40 is a schematic illustration showing an exemplary construction ofa package according to the modified optical dispersion compensationsystem shown in FIG. 39;

FIG. 41 is a block diagram showing an optical dispersion compensationsystem to which an optical dispersion compensation method of a fifteenthpreferred embodiment of the present invention is applied;

FIG. 42 is a block diagram showing an adaptation of the opticaldispersion compensation system shown in FIG. 41; and

FIG. 43 is a block diagram showing another adaptation to the opticaldispersion compensation system shown in FIG. 41.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A. First Embodiment

FIGS. 1 to 4 illustrate an optical wavelength multiplex transmissionmethod according to a first preferred embodiment of the presentinvention.

First, an optical WDM distribution transmission system to which theoptical wavelength multiplex transmission method of the presentembodiment is applied will be described. Referring to FIG. 2, theoptical WDM distribution transmission system shown includes atransmission circuit 1 which multiplexes signals from a plurality ofchannels into signal light waves having different frequencies orwavelengths in a high density.

The transmission circuit 1 includes a laser diode (LD-1 to LD-n) 1 aprovided for each of the channels CH-1 to CH-n, and a wave combiner 1 bfor receiving signal light waves from the laser diodes 1 a of thechannels and multiplexing the received signal light waves.

The optical WDM distribution transmission system further includes anoptical fiber 2 for transmitting multiplexed signal light waves from thetransmission circuit 1, a distributor 3 for distributing a signal fromthe optical fiber 2 among different channels, and a reception circuit 4provided for each of the channels CH-i (i=1 to n) for receiving signallight of a frequency or wavelength allocated to the channel. Each of thereception circuits 4 includes an optical filter 4 a for extracting andoutputting a corresponding signal from multiplexed signal light, acontrol circuit 4 b for controlling the optical filter 4 a, and adetector 4 c for detecting signal light from the optical filter 4 a.

By the way, FWM as a non-linear effect of the optical fiber 2 is aphenomenon which is produced by optical frequency mixing betweendifferent signal light waves having different frequencies or wavelengthsfrom each other when the signal light waves are multiplexed and inputtedto the optical fiber 2 using a band in the proximity of thezero-dispersion wavelength of the optical fiber 2, and makes a factor ofcrosstalk from another channel and degrades the signal transmissioncharacteristic.

The FWM which is a non-linear effect of the optical fiber 2 has a mostsignificant influence upon optical WDM (FDM) transmission which employsa band in the proximity of the zero-dispersion wavelength of the opticalfiber 2. In order to give a more detailed description of the FWM, asystem design which must be performed taking an influence of the FWMinto consideration, particularly, the channel spacing, the channelarrangement and the input power, will be described below.

For example, when signal light waves of frequencies f₁ to f_(n)(wavelengths λ₁ to λ_(n)) are inputted, a fourth light wave of afrequency f_(ijk) (wavelength λ_(ijk); i≠k, j≠k) is generated fromarbitrary three waves f_(i), f_(j) and f_(k) (wavelengths λ_(i), λ_(j)and λ_(k)) of the signal light waves in accordance with the third-ordernon-linear susceptibility χ₁₁₁₁ of the optical fiber 2, and makes an FWMwave. The FWM wave of the frequency f_(ijk) (frequency λ_(ijk)) appearsat the position of an optical frequency which satisfies the followingequation (1). When the number of channels is great with an equalfrequency spacing, several FWM waves appear at the positions of thefrequencies f_(ijk) (wavelengths λ_(ijk)) according to combinations ofi, j and k and are superimposed on signal light waves. Consequently, thecrosstalk is further degraded.

f_(ijk)=f_(i)+f_(j)−f_(k)(λ_(ijk)=λ_(i)+λ_(j)−λ_(k))  (1)

Meanwhile, the frequency f_(ijk) (wavelength λ_(ijk)) exhibits a highoccurrence efficiency in the proximity of the zero-dispersionwavelength, and the efficiency is varied by the phase relationship amongthe frequencies f_(i), f_(j), f_(k) and f_(ijk) (wavelengths λ_(i),λ_(j), λ_(k) and λ_(ijk)), or the efficiency becomes higher as the phaseinconsistency amount Δβ, which will be hereinafter described, increases.

Generally, where polarization conditions of three signal channelscoincide with each other, the optical power P_(ijk) of an FWM wave isgiven by the following equation (2):

P_(ijk)=η_(ijk)·{(1,024π⁶·χ₁₁₁₁²·d²)/n⁴·λ²·c²}·(L_(eff)/A_(eff))²·P_(i)·P_(j)·P_(k)·exp(−αL)  (2)

where η_(ijk) is the occurrence efficiency of the frequency f_(ijk)(wavelength λ_(ijk)) λ₁₁₁₁ is the third-order non-linear susceptibility,d is the degeneracy coefficient (d=6 when i≠j≠k, and d=3 when i=j≠k), nis the refraction index of the core, λ is the signal wavelength, c isthe velocity of light, L_(eff) is the effective optical fiber lengthgiven by the equation (3) given below, A_(eff) is the effective corearea (=πW², W is the mode field diameter), α is the attenuationcoefficient of the optical fiber, and P_(i), P_(j) and P_(k) are theinput powers of signal light waves of the frequencies f_(i), f_(j) andf_(k) (wavelengths λ₁, λ_(j) and λ_(k)), respectively.

L_(eff)={1−exp(−αL)}/α  (3)

where the occurrence efficiency η_(ijk) (=η) is given by the followingequation (4):

η=α²·[1+4exp(−αL)·sin²(ΔβL/2)/{1−exp(−αL)}²]/(α²+Δβ²)  (4)

where L is the optical fiber length, and Δβ is the phase inconsistencyamount. Further, if it is assumed that the dispersion slope dD/dλ of theoptical fiber 2 is fixed with respect to the wavelength, the phaseinconsistency amount Δβ is given by the equation (5) or (6) below:

a. In the case of f_(i)≠f_(j)≠f_(k) (λ_(i)≠λ_(j)≠λ_(k));

Δβ=(πλ⁴/3c²)·(dD/dλ)·{(f_(i)+f_(j)−f_(k)−f₀)³−(f_(i)−f₀)³−(f_(j)−f₀)³+(f_(k)−f₀)³)}  (5)

b. In the case of f_(i)=f_(j)≠f_(k) (λ_(i)=λ_(j)≠λ_(k));

Δβ=(πλ⁴/c²)·(dD/dλ)·2·(f_(i)−f₀)·(f_(i)−f_(k))²  (6)

where D is the chromatic dispersion of the optical fiber, dD/dλ is thechromatic dispersion of the second order of the optical fiber, and f₀ isthe zero-dispersion optical frequency. It is to be noted that theequations (5) and (6) stand also where the frequencies f_(i), f_(j),f_(k) and f₀ are replaced by the wavelengths λ_(i), λ_(j), λ_(k) and λ₀,respectively.

Where a plurality of channels are involved, combinations of i, j and kof FWM waves which appear at the positions of the frequency f_(ijk)(wavelength λ_(ijk)) are calculated, and optical powers P_(ijk) areindividually calculated for them. Then, the sum total of the opticalpowers P_(ijk) makes an optical power of the FWM wave produced at theposition of the frequency f_(ijk) (wavelength λ_(ijk)). Using the sumtotal of the optical powers, a crosstalk amount CR is calculated inaccordance with the following equation (7);

CR=10·log{(sum total of all FWM optical powers appearing at positions off_(ijk))/(signal optical power at positions of f_(ijk))}  (7)

The influence of FWM can be estimated using the equations (2) and (4) to(7), which allows designing of values of parameters of the system suchas a channel spacing, a channel arrangement and an input power. In thedescription of action and effects of the first to sixth embodiments ofthe invention given below, an estimated influence of FWM (refer to FIG.3, 4, 7, 11 or 12) obtained in accordance with the equations givenhereinabove will be used suitably.

As described hereinabove, in order to prevent waveform deterioration bydispersion of the optical fiber 2, it is necessary to make use of a bandin the proximity of the zero-dispersion wavelength of the optical fiberand also to assure a channel spacing and a signal channel arrangementtaking an influence of FWM, which appears significantly when the band isused for multiplexing, into consideration. To this end, in the opticalwavelength multiplex transmission method according to the firstembodiment of the present invention, signal light waves of differentchannels are arranged, for example, as illustrated in FIG. 1.

According to such a channel arrangement as illustrated in FIG. 1, an FWMsuppressing guard band 5 of a fixed width ranging from a zero-dispersionwavelength λ₀ is provided, and signal light waves are disposed on thelonger wavelength side 6 than the zero-dispersion wavelength λ₀ outsidethe guard band 5.

Due to the construction described above, in the optical wavelengthmultiplex transmission system to which the optical wavelength multiplextransmission method according to the first embodiment of the presentinvention is applied, signals from the different channels aremultiplexed in a high density as signal light waves of differentfrequencies or wavelengths from one another by the transmission circuit1 and transmitted by way of the optical fiber 2.

The signal light waves transmitted by way of the optical fiber 2 aredemultiplexed by the distributor 3 and inputted to the receptioncircuits 4 of the corresponding channels and detected as signal lightwaves of the frequencies or wavelengths corresponding to the inputchannels.

In this instance, where the system is constructed, for example, suchthat the number of channels of the transmission circuit 1 is 16 (n=16);the channel spacing is 150 GHz; the length L of the optical fiber 2 is90 km; and the optical input power P of each channel is +3 dBm, theresults of calculation of crosstalk amounts of the channels are such asillustrated in FIG. 3. The parameters used for the calculation areχ₁₁₁₁=5.0×10⁻¹⁵ cm³/erg(esu), A_(eff)=4.6×10⁻¹¹ m², α=5.2958×10⁻⁵ m⁻¹(0.23 dB/km), and dD/dλ=0.065 ps/(km·nm²).

In FIG. 3, such a representation as “0.0 ps/nm/km” indicates a value ofdispersion at the channel 1 CH1. As the channel number (CH No.)increases, the dispersion value increases in accordance with thedispersion slope dD/dλ. From the results illustrated in FIG. 3, thecrosstalk amounts at the channels CH2, CH3 and CH4 exhibit comparativelyhigh values.

Results of calculation performed paying attention to the channels CH2,CH3 and CH4 are illustrated in FIG. 4. As seen from FIG. 4, in order tosuppress the crosstalk amount, for example, below 30 dB, the dispersionvalue of the channel CH1 should be 0.64 ps/nm/km or more, and where, forexample, dD/dλ=0.065 ps/(km·nm²), the channel CH1 should be displacedapproximately 10 nm from the zero-dispersion wavelength λ₀. Accordingly,the guard band 5 should have a width of 10 nm.

In this manner, according to the optical wavelength multiplextransmission method of the first embodiment, by arranging signal lightwaves of different channels from the zero-dispersion wavelength λ₀ ofthe optical fiber 2 with the guard band 5 interposed therebetween, aninfluence of FWM can be suppressed and an influence from another channelby crosstalk can be suppressed. Further, since the band can be utilizedefficiently, an optical communication system of an increased capacitycan be realized while maintaining a high degree of transmissionaccuracy.

It is to be noted that, while, in the present embodiment, signal lightwaves are arranged on the longer wavelength side 6 with respect to thezero-dispersion wavelength λ₀, they may alternatively be arranged on ashorter wavelength side 7 with respect to the zero-dispersion wavelengthλ₀ with the guard band 5 interposed therebetween.

B. Second Embodiment

Subsequently, an optical wavelength multiplex transmission methodaccording to a second preferred embodiment of the present invention willbe described. FIG. 5 illustrates an arrangement of signal light waves ofa plurality of channels of the optical wavelength multiplex transmissionmethod. It is to be noted that also the optical wavelength multiplextransmission method of the second embodiment is applied to a systemsimilar to the optical WDM (FDM) distribution transmission systemdescribed hereinabove with reference to FIG. 2, and overlappingdescription of the same will be omitted herein to avoid redundancy.

In the optical wavelength multiplex transmission method according to thepresent embodiment, a pair of FWM suppressing guard bands 5 are providedon the opposite sides of the zero-dispersion wavelength λ₀, and signallight waves of different channels are arranged on the shorter wavelengthside 7 and the longer wavelength side 6 outside the guard bands 5.

Due to the channel arrangement described above, with the opticalwavelength multiplex transmission method of the second embodiment, evenif the zero-dispersion wavelength λ₀ is positioned within a band of anoptical amplifier or of an optical part, signal light waves can bearranged efficiently and compactly while suppressing an effect of FWM inthe limited band to suppress an influence from another channel bycrosstalk, and accordingly, there is an advantage in that an increase ofthe capacity of the system can be realized while maintaining a highdegree of transmission accuracy.

C. Third Embodiment

Subsequently, an optical wavelength multiplex transmission methodaccording to a third preferred embodiment of the present invention willbe described. FIG. 6 illustrates an arrangement of signal light waves ofa plurality of channels of the optical wavelength multiplex transmissionmethod, and FIG. 7 illustrates operation according to the opticalwavelength multiplex transmission method. It is to be noted that alsothe optical wavelength multiplex transmission method of the thirdembodiment is applied to a system similar to the optical WDM (FDM)distribution transmission system described hereinabove with reference toFIG. 2, and overlapping description of the same will be omitted hereinto avoid redundancy.

According to the optical wavelength multiplex transmission method of thethird embodiment, as shown in FIG. 6, a pair of FWM suppressing guardbands 5 are provided in an asymmetrical relationship on the shorterwavelength side 7 and the longer wavelength side 6 with respect to thezero-dispersion wavelength λ₀, and signal light waves to be multiplexedare arranged such that the channel spacing thereof is set differentbetween the shorter wave side 7 (Δf) and the longer wavelength side 6(Δf′).

Since the channel spacing is set different between the shorter andlonger wavelength sides (with respect to the guard bands 5), it can beprevented that the position at which FWM light appears between signallight on the shorter wavelength side and signal light on the longerwavelength side (with respect to the guard bands) coincides with somesignal light wavelength, and consequently, an influence from anotherchannel by crosstalk is suppressed. Here, the width by which the band inwhich FWM light may appear is displaced from the band of the signallight is desirably set within a range within which the width can besuppressed by the optical filter 4 a on the reception side.

Where the channel spacing is made different between the left and theright such that it is set, for example, as shown in FIG. 7, to 200 GHzon the shorter wavelength side 7 and to 150 GHz on the longer wavelengthside 6 and the width of the guard band 5 is set to 1.6 nm on the shorterwavelength side 7 and to 4 nm on the longer wavelength side 6, FWM lightis produced between different channels, but production of FWM light isreduced within the bands of signal light and also the crosstalk amountis reduced.

In this manner, also with the optical wavelength multiplex transmissionmethod of the third embodiment, since signal light waves of differentchannels are arranged on the opposite sides of the zero-dispersionwavelength λ₀ in a spaced relationship from the zero-dispersionwavelength λ₀ with the guard bands 5 interposed between them, aninfluence of FWM can be suppressed and an influence from another channelby crosstalk can be suppressed. Further, since the band can be utilizedefficiently, there is an advantage in that an increase of the capacityof the system can be realized while maintaining a high degree oftransmission accuracy.

D. Fourth Embodiment

Subsequently, an optical wavelength multiplex transmission methodaccording to a fourth preferred embodiment of the present invention willbe described. FIG. 8 illustrates an arrangement of signal light waves ofa plurality of channels of the optical wavelength multiplex transmissionmethod. It is to be noted that also the optical wavelength multiplextransmission method of the fourth embodiment is applied to a systemsimilar to the optical WDM (FDM) distribution transmission systemdescribed hereinabove with reference to FIG. 2, and overlappingdescription of the same will be omitted herein to avoid redundancy.

In the optical wavelength multiplex transmission method of the fourthembodiment, the channel spacings on the shorter wavelength side 7 andthe longer wavelength side 6 are set individually to a constantmultiplied by different integral numbers as seen from FIG. 8.

If, for example, the channel spacing Δf is Δf=A·X, then the channelspacing between a channel n+4 and another channel n+5 is set to Δf′=B·X,and the channel spacing between a channel n+m−1 and another channel n+mis set to Δf″=C·X. Here, X is the constant, and A, B and C are theintegral numbers.

Further, as seen from FIG. 8, also in the present embodiment, the FWMsuppression guard bands 5 are arranged asymmetrically on the shorterwavelength side 7 and the longer wavelength side 6 with respect to thezero-dispersion wavelength λ₀.

In the transmission circuit 1 shown in FIG. 2; it is required tostabilize the wavelengths of the laser diodes 1 a in a desired channelarrangement and at a desired channel spacing, while in the receptioncircuit 4, it is required to select and extract a channel. The channelarrangement and the channel spacing required in order to suppress suchan influence of FWM as described above are desired to be easy to controlby the transmission circuit 1 and easy to extract by the receptioncircuit 4.

Generally, control of the channel spacing is performed making use of aperiodic characteristic of an optical interferometer. When it is tried,for example, to perform control of the channel spacing using Fabry-Perotinterferometers, if the desired channel spacing is equal to the distancebetween transmission peaks of the Fabry-Perot interferometers or equalto an integral number of times the distance between such transmissionpeaks, then if the wavelengths of the individual laser diodes 1 a arestabilized at the positions of the transmission peaks using one of theFabry-Perot interferometers as a reference, control of all of thechannels can be realized simply. However, where the channels arearranged at different spacings, control is complicated.

From such point of view, by setting the channel spacings on the shorterwavelength side 7 and the longer wavelength side 6 (with respect to thezero-dispersion wavelength λ₀) to integral numbers of times a constant(distance of one period of transmission peaks of optical interferometersor an integral number of times the distance), channels on the shorterwavelength side 7 and the longer wavelength side 6 can be controlled byone or two Fabry-Perot interferometers of the same characteristic. Thissimilarly applies to the reception circuit 4. In particular, by settingthe channel spacings to integral numbers of times a constant, aninterferometer of the same characteristic can be used.

In this manner, with the optical wavelength multiplex transmissionmethod of the fourth embodiment, since the channel spacings on thetransmission side can be controlled by means of a single or twoFabry-Perot interferometers, there is an advantage in that the controlon the transmission side can be simplified.

This also applies to the reception side. In particular, by setting thechannel spacings to integral numbers of times a constant, aninterferometer having the same characteristic as that of theinterferometers on the transmission side can be used. Consequently,there is an advantage in that selective reception is facilitated and theapparatus can be simplified.

It is to be noted that, in the present embodiment, the channel spacingbetween adjacent channels of signal light waves of a plurality ofchannels can be set such that it may be different on the shorterwavelength side 7 and the longer wavelength side 6 outside the guardbands 5.

E. Fifth Embodiment

Subsequently, an optical wavelength multiplex transmission methodaccording to a fifth preferred embodiment of the present invention willbe described. FIG. 9 illustrates an arrangement of signal light waves ofa plurality of channels of the optical wavelength multiplex transmissionmethod. It is to be noted that also the optical wavelength multiplextransmission method of the fifth embodiment is applied to a systemsimilar to the optical WDM (FDM) distribution transmission systemdescribed hereinabove with reference to FIG. 2, and overlappingdescription of the same will be omitted herein to avoid redundancy.

In the optical wavelength multiplex transmission method of the fifthembodiment, the frequencies or wavelengths of signal light waves ofdifferent channels are set such that the spacings between the signallight waves of the channels arranged on the opposite sides of the FWMsuppression guard bands 5 may satisfy the relationship wherein thesignal light waves are spaced from each other by spacings equal tointegral numbers of times a constant on the opposite sides of the guardbands 5.

In particular, where the optical frequency of the channel CHi isrepresented by f, the optical frequency of an arbitrary channel j is setso as to satisfy f±A·X, where A is an integral number and X is aconstant.

Due to the channel arrangement described above, with the opticalwavelength multiplex transmission method of the fifth embodiment, thechannel spacings on the opposite sides of the guard bands 5 can be setto integral numbers of times a constant (distance of one period oftransmission peaks of an optical interferometer or an integral number oftimes the distance), and consequently, control of the channel spacingson the transmission side can be realized only with a single opticalinterferometer. Further, since it is only required to use aninterferometer of the same characteristic on the reception side, thereis an advantage in that selective reception is facilitated and theapparatus is simplified.

F. Sixth Embodiment

Subsequently, an optical wavelength multiplex transmission methodaccording to a sixth preferred embodiment of the present invention willbe described. FIG. 10 illustrates an arrangement of signal light wavesof a plurality of channels of the optical wavelength multiplextransmission method, and FIGS. 11 and 12 illustrate operation of thesame. It is to be noted that also the optical wavelength multiplextransmission method of the sixth embodiment is applied to a systemsimilar to the optical WDM (FDM) distribution transmission systemdescribed hereinabove with reference to FIG. 2, and overlappingdescription of the same will be omitted herein to avoid redundancy.

In the optical wavelength multiplex transmission method of the sixthembodiment, different channels are arranged such that two or morechannels may not overlap with each other, that is, one pair of channelsor less may have an equal absolute value of a dispersion value when thechannel arrangement is folded on itself at the zero-dispersionwavelength λ₀ as viewed on the optical frequency (optical wavelength)axis as seen in FIG. 10. In the arrangement shown in FIG. 10, only onepair of channels CH3 and CH8 overlap with each other.

Where, for example, the channel number of the transmission circuit 1 is16; the channel spacings are 150 GHz, 200 GHz and 250 GHz; the length Lof the optical fiber 2 is 90 km; and the optical input power P per onechannel is 0 dBm, results of calculation of crosstalk of differentchannels are such as illustrated in FIG. 11, and in the case of anothersystem wherein the channel number of the transmission circuit 1 is 16;the channel spacings are 150 GHz, 200 GHz and 300 GHz; the length L ofthe optical fiber 2 is 90 km; and the optical input power P per onechannel is 0 dBm, results of calculation of crosstalk of differentchannels are such as illustrated in FIG. 12. As seen from FIG. 11 or 12,all of the channels exhibit good values of cross talk around −30 dB.

While an influence of crosstalk at the channel CH2, CH5, CH11 or CH15can be seen in FIG. 7 which illustrates operation of the thirdembodiment, if the channel arrangement illustrated in FIG. 7 is foldedon itself at the zero-dispersion wavelength λ₀, the channels CH2 andCH15 overlap with each other and the channels CH5 and CH11 overlap witheach other. In other words, the two pairs of channels have equalabsolute values of dispersion values. In contrast, in the presentembodiment, by setting the channel arrangement so that only one pair ofchannels may be allowed to have an equal absolute value of a dispersionvalue as seen in FIGS. 11 and 12, crosstalk exhibits good values aroundapproximately −30 dB with all of the channels as described hereinabove.

Where two or more pairs of channels have equal absolute values ofdispersion values on the opposite sides of the zero-dispersionwavelength λ₀, as can be seen also from the equation (5) givenhereinabove, the phase mismatch amount Δβ exhibits the value 0 with acombination of three channels within two pairs of channels, and FWMlight appears in a high efficiency at the position of the remaining onechannel. After all, FWM light appears at the optical frequency positionsof all of the four channels of the two pairs and degrades the crosstalk.Accordingly, the channels are set such that less than two pairs ofchannels may have an equal absolute value of a dispersion value.

In this manner, with the optical wavelength multiplex transmissionmethod of the sixth embodiment, since less than two pairs of channelshave an equal value of a dispersion value on the opposite sides of thezero-dispersion wavelength λ₀, production of FWM light can besuppressed, and an influence from another channel by crosstalk can besuppressed with certainty. Further, since a band can be utilizedefficiently similarly as in the first to fifth embodiments describedabove, there is an advantage in that an increase of the capacity of thesystem can be realized while maintaining a high degree of transmissionaccuracy.

It is to be noted that, while, in the first to sixth embodimentsdescribed above, the channel spacing is set in terms of a frequency, itmay alternatively be set in terms of a wavelength, and also in thisinstance, similar advantages to those of the embodiments described abovecan be achieved.

G. Seventh Embodiment

In order to suppress and eliminate crosstalk by FWM between signal lightwaves in an optical transmission system based on the WDM method whichemploys a band around the zero-dispersion wavelength of an optical fiber(in a seventh preferred embodiment of the present invention), such anoptical amplifier multi-repeater system (regenerative repeater system)as described hereinbelow with reference to FIG. 15), it is required toseparate a signal light band and the zero-dispersion wavelength of theoptical fiber from each other as described hereinabove. The channelarrangement then depends principally upon a guard band for suppressionof FWM (guard band or bands 5 described in the first to sixthembodiments), a limiting band by an SPD-GVD effect and a gain band of anEDFA. Meanwhile, the zero-dispersion wavelength of an actual opticalfiber exhibits a deviation in its longitudinal direction, and it is veryimportant for designing of a system to control the zero-dispersionwavelength and the deviation of the zero-dispersion wavelength. Further,the apparent zero-dispersion wavelength can be shifted by employment ofan optical dispersion compensator, which provides an advantage to allowthe deviation of the zero-dispersion wavelength.

In the seventh and eighth embodiments described below, a channelarrangement method according to the WDM method when the factorsdescribed above are taken into consideration will be described.Conversely speaking, this can be regarded as a defining method betweenthe zero-dispersion wavelength of an optical fiber and the deviation ofthe zero-dispersion wavelength in a situation wherein the number ofchannels and the channel spacing are decided.

In the following description, a limiting band by a. a wavelengthmultiplex signal band, b. a gain band of an EDFA, c. a guard band forsuppression of FWM and d. an SPM-GVD effect, which are factors to limitthe signal light band, will be described first, and then therelationship between a channel arrangement and characteristics requiredfor an optical fiber will be described taking presence or absence of aninserted optical dispersion compensator into consideration.

·Limiting Factors

a. Wavelength Multiplex Signal Band

Where signal light of n waves is arranged at an equal wavelength spacing(channel spacing) Δλ_(S), a wavelength multiplex signal light bandΔλ_(WDM) is given by Δλ_(S)×(n−1). It is to be noted that, in the caseof an equal wavelength spacing arrangement, FWM light in the signallight band is liable to become high while wavelength stabilization isfacilitated as described hereinabove in the fourth and fifthembodiments.

b. EDFA Gain Band

In the case of optical transmission of the WDM method, in order to makethe reception characteristic equal among different waves, the signallight power must be made equal among the different waves, and to thisend, a frequency band in which the gain of the EDFA exhibits a flatcharacteristic must be used. For example, in FIG. 16, an example of anASE spectrum after EDFAs are connected at four stages (the ASE spectrumdistribution is substantially equal to the gain distribution of an EDFA)is illustrated, and in the EDFA technique at present, the range of 1,550to 1,560 nm is a frequency band in which the gain is flat. Consequently,it is desirable to arrange signal light of all channels within thebandwidth (Δλ_(EDFA)=10 nm).

It is to be noted that, as another frequency band than that describedabove, a frequency band in the proximity of 1,535 nm at which the gainis equal may be used. It is to be noted that, as factors to decide thechannel spacing then, there are a wavelength selective filtercharacteristic, wavelength stability of a semiconductor laser and soforth. Further, as means for expanding the gain bandwidth Δλ_(EDFA) ofthe EDFA, optimization of an EDFA operation point, optimization ofcomposition of the EDF, insertion of an optical notch filter and soforth may be available.

c. Guard Band for FWM Suppression

As described also in the first embodiment, in optical WDM (FDM)transmission which employs a band in the proximity of thezero-dispersion wavelength of an optical fiber, it is required to set achannel spacing, a channel arrangement and an input power taking aninfluence of FWM into consideration. For example, when signal lightwaves of, for example, wavelengths λ₁, to λ_(n) are inputted, a fourthlight wave (FWM wave) of the wavelength λ_(ijk) (i≠k, j≠k) is producedfrom arbitrary three λ_(i), λ_(j) and λ_(k) of the input signal lightwaves by a third-order non-linear susceptibility χ₁₁₁₁ of the opticalfiber.

The wavelength λ_(ijk) satisfies the relationship of the equation (1)given hereinabove and causes crosstalk and degrades the transmissioncharacteristic when signal light is present at the position.Particularly where the channel spacings are equal and the number ofchannels is great, a plurality of FWM waves are overlapped at thepositions of wavelengths λ_(ijk) by combinations of i, j and k,resulting in increase of the crosstalk amount. Further, the productionefficiency η_(ijk) of the wavelength λ_(ijk) varies depending upon thephase relationship among the wavelengths λ_(i), λ_(j), λ_(k) and λ_(ijk)and indicates a high value in the proximity of the zero-dispersionwavelength λ₀ of the optical fiber.

Generally, when the polarization conditions of three signal light wavesand the phases of the three signal light waves at the input terminal ofan optical fiber, the FWM optical powers P_(ijk) and the productionefficiencies η_(ijk) are given by the equations (2) and (3) and theequations (4) to (6) given hereinabove.

An example of calculation of crosstalk amounts (refer to the equation(7) given hereinabove) of different channels where 16 signal light wavesare arranged at an equal distance of the wavelength spacing Δλ_(S)=1.2nm as shown, for example, in FIG. 17 and when the dispersion valueD_(ch1) of the channel 1 is varied is illustrated in FIG. 18. Parametersused for the calculation are: λ=1.55 μm, χ₁₁₁₁=5.0×10⁻¹⁵ esu,A_(eff)=4.6×10⁻¹¹ m², α=5.3×10⁻⁵ m⁻¹ (0.23 dB/km), dD/dλ=0.065ps/(km·nm²), L=90 km, and P_(i)=0 dBm/ch.

As seen from FIG. 18, the number of combinations of FWM light wavesoverlapped with different channels exhibits its maximum value with thechannel 7 or 8 at the center. However, since the dispersion values atthe different channels are different, the crosstalk amount exhibits amaximum level with the channels 2 to 4 (this is a similar result to thatdescribed hereinabove with reference to FIG. 3 in connection with thefirst embodiment). Where the required crosstalk amount is −30 dB, thedispersion value D_(chi) of the channel 1 must necessarily be set to0.25 ps/(km·nm). In other words, the wavelength spacing between thezero-dispersion wavelength λ₀ and the wavelength λ₁ of the channel 1must necessarily be set to 3.8 nm or more, and this will be hereinafterreferred to as FWM suppressing guard band Δλ_(g) in the presentembodiment.

d. Limiting Band by SPM-GVD Effect

FIG. 15 shows the construction of a regenerative repeater system oroptical transmission system to which the optical wavelength multiplextransmission method of the seventh embodiment of the present inventionis applied. Referring to FIG. 15, the regenerative repeater system shownincludes a transmitter 11 for converting an electric signal into anoptical signal or signal light and performing optical wavelengthmultiplexing using the construction (transmission circuit 1) describedhereinabove with reference to FIG. 2, and a plurality of in-lineamplifiers 12 inserted substantially at a fixed distance L_(in-line) inan optical transmission line (optical fiber 2) for amplifying a signalattenuated by line loss.

The regenerative repeater system further includes a plurality ofregenerative-repeaters 13 interposed substantially at a fixed distanceL_(R-rep) greater than the distance L_(in-line) between the in-linerepeaters 12 in the optical transmission line (optical fiber 2). Theregenerative-repeaters 13 are provided to regenerate pulse signals fromsignal light in the optical transmission line before the signal light isdegraded by an influence of noise relying upon the line characteristicinto a condition in which signals thereof cannot be discriminated fromone another, and have three functions represented by words beginningwith R including reshaping, retiming and regenerating. Therefore, such aregenerative-repeater is also called 3R repeater.

The regenerative repeater system further includes a receiver 14 fordemultiplexing signal light, which has been multiplexed by theconstruction (reception circuit 4) described hereinabove with referenceto FIG. 2 and converting the signal light waves obtained by thedemodulation into electric signals.

In the present embodiment, the transmitter 11 and the receiver 14 areinterconnected by way of the optical fiber 2 with the plurality ofin-line repeaters 12 and regenerative-repeaters 13 interposed in theoptical fiber 2 to construct the optical transmission system orregenerative-repeater system 10 according to the optical amplifiermulti-repeater WDM method.

By the way, in the case of the optical transmission system 10 of such aconstruction as described above, the distance L_(R-rep) between theregenerative-repeaters 13 are restricted principally by two factorsincluding 1. optical signal to noise degradation by ASE accumulation atthe in-line repeaters 12 and 2. waveform degradation by an SPM-GVDeffect caused by a Kerr-effect in the optical fiber 2. Simultaneously,the lower limit to the input power into the optical fiber 2 is limitedby the optical signal to noise ratio while the upper limit is limited bythe SPM-GVD effect. It is to be noted that, for evaluation of.waveformdegradation by an SPM-GVD effect, generally a simulation which involvessolution of a non-linear Schroedinger equation using the split stepFourier method is effective.

FIG. 19 illustrates an example of a relationship between the input powerto the optical fiber 2 and the distance L_(R-rep) between theregenerative-repeaters 13 when the transmission rate is 10 Gbps, thedistance L_(in-line) between the in-line repeaters 12 is 70 km and onlyone wave is transmitted. If it is assumed that the variation of theoptical output from each optical amplifier (in-line repeater 12) is ±2dB, where an allowable dispersion value D_(allow) is D_(allow)=±1ps/(nm·km), the maximum value of the distance L_(R-rep) between theregenerative-repeaters 13 is 280 km, and where the allowable dispersionvalue D_(allow) is D_(allow)=±2 ps/(nm·km), the maximum value of thedistance L_(R-rep) between the regenerative-repeaters 13 is 210 km. Inorder to realize long-haul transmission, it is necessary to set theallowable dispersion value low and set the input power to the opticalfiber 2 high.

·Relationship between Channel Arrangement and Characteristic Requiredfor Optical Fiber

Three required characteristics for a DSF (optical fiber 2) must be takeninto consideration when it is tried to achieve optical transmissionbased on the WDM method, including 1. the zero-dispersion wavelength λ₀,2. the zero-dispersion wavelength deviation ±Δλ₀, and 3. the dispersionslope (second-order dispersion) dD/dλ as described hereinabove. Here,the zero-dispersion wavelength deviation ±Δλ₀ signifies not only adispersion involved in production of a DSF but also a maximum deviationwidth of the zero-dispersion wavelength λ₀ in the longitudinal directionof the optical fiber 2 within the distance L_(R-rep) between theregenerative-repeaters 13.

FIG. 20 illustrates a result of measurement of the FWM productionefficiency η when two signal light waves were inputted to an actual DSFand the wavelength λ₂ of one of the two signal light waves was fixed to1,557 nm while the wavelength λ₁ of the other signal light wave wasvaried. In FIG. 20, the result of measurement is indicated by a solidline interconnecting blank round marks. In the measurement, the opticalfiber length was 60 km, and the power of each signal light wave was +4dBm. Comparison with another result of calculation (indicated by abroken line in FIG. 20) conducted with the zero-dispersion wavelength λ₀fixed to a fixed value reveals that the measurement values indicated bya solid line in FIG. 20 are distributed over a wider wavelength range.This signifies that the zero-dispersion wavelength λ₀ of the actual DSFexhibits a deviation in the longitudinal direction of the DSF.

Taking the foregoing points described above into consideration, in theoptical wavelength multiplex transmission method of the seventhembodiment of the present invention, signal light waves of differentchannels are arranged in such a manner, for example, as illustrated inFIG. 13. It is to be noted that, in the present embodiment, descriptionwill be given of the case wherein signal light waves of four channelsare wavelength multiplexed and transmitted.

In particular, referring to FIG. 13, taking the zero-dispersionwavelength λ₀ of the optical fiber 2 and the zero-dispersion wavelengthdeviation ±Δλ₀ in the longitudinal direction of the optical fiber 2 intoconsideration, signal light waves of four channels to be multiplexed arearranged at an equal spacing Δλ_(S) on the shorter wavelength side thana short wavelength end λ₀−Δλ₀ of the range of the zero-dispersionwavelength deviation of the optical fiber 2.

In this instance, an FWM suppressing guard band Δλ_(g) is provided onthe shorter wavelength side than the short wavelength end λ₀−Δλ₀ of thezero-dispersion wavelength deviation range of the optical fiber 2, andsignal light waves of four channels (wavelengths λ₁ to λ₄ in thechannels 1 to 4) are arranged on the further shorter wavelength sidethan the wavelength λ₀−Δλ₀−Δλ_(g). In the present embodiment, thewavelength λ₁ of the channel 1 is set to the position spaced byΔλ₀+Δλ_(g) on the shorter wavelength side than the zero-dispersionwavelength λ₀ of the DSF (optical fiber 2). In other words, thewavelength λ₀−Δλ₀−Δλ_(g) is set so as to coincide with the wavelength λ₁of the channel 1.

Further, in the present embodiment, signal light waves of four channelsare arranged within the transmissible bandwidth Δλ_(SPM-GVD) defined bythe allowable dispersion value D_(allow) determined from an SPM-GVDeffect in the optical fiber 2. In particular, as seen from FIG. 13, thetransmissible wavelength range of signal light is a range withinΔλ_(SPM-GVD)=|D_(allow)|/(dD/dλ) on the shorter wavelength side than thelong wavelength end λ₀+Δλ₀ of the zero-dispersion wavelength deviationrange of the optical fiber 2. In this instance, in order to allowtransmission of four waves and allow the zero-dispersion wavelengthdeviation Δλ₀ (to be set as great as possible, the wavelengthλ_(SPM-GVD) (=(λ₀+Δλ₀) Δλ_(SPM-GVD)) and the wavelength λ₄ of thechannel 4 are set so as to coincide with each other.

Further, in the present embodiment, signal light waves of four channelsare arranged within a gain band Δλ_(EDFA) (such a range of 1,550 to1,560 nm as illustrated, for example, in FIG. 16) of an EDFA (opticalamplifier arranged in each in-line repeater 12) connected to the opticalfiber 2.

It is to be noted that, though not illustrated in FIG. 13, when theproductivity of semiconductor lasers (light sources of signal lightwaves) and/or the optical wavelength variations of signal light wavescaused by the wavelength control accuracy are taken into consideration,the bandwidth Δλ_(WDM) within which signal light waves of a plurality ofchannels are arranged is set in an expanded condition in accordance withsuch variations.

Here, the example of a signal light arrangement illustrated in FIG. 13is described in more detail by way of an example of numerical values.The relationship between a channel arrangement and characteristicsrequired for a DSF is described for the case wherein, for example, foursignal light waves of the transmission rate 10 Gbps are arranged at anequal distance of the wavelength spacing Δλ_(S)=2 nm on the shorterwavelength side than the zero-dispersion wavelength λ₀ of the DSF(optical fiber 2) and the distance L_(in-line) between the in-linerepeaters 12 is set to 70 km while the distance L_(R-rep) between theregenerative-repeaters 13 is set to 280 km.

First, the relationship of the guard band Δλ_(g) with which thecrosstalk amounts at all of the channels are smaller than −30 dB to thewavelength spacing Δλ_(S) when the optical fiber length is 70 km and theinput power of each channel is +6 dBm is illustrated in FIG. 21. FromFIG. 21, it can be seen that, where the wavelength spacing Δλ_(s), is 2nm (signal light bandwidth Δλ_(WDM)=6 nm), the guard band Δλ_(g) isrequired to be Δλ_(g)=3 nm.

In order to effectively utilize the gain band (1,550 to 1,560 nm) of theEDFA, the wavelength λ₁ of the channel 1 is set to 1,560 nm which is thelonger wavelength end of the gain band as seen from FIG. 13. In thisinstance, the wavelength λ₁ is displaced by Δλ₀+Δλ_(g) toward theshorter wavelength side from the zero-dispersion wavelength λ₀ of theDSF as described hereinabove.

Further, since the allowable dispersion value D_(allow) with which thedistance L_(R-rep) between the regenerative-repeaters 13 isL_(R-rep)=280 km is −1 ps/(nm·km) from FIG. 19, the transmissible signallight wavelength range is a range withinΔλ_(SPM-GVD)=|D_(allow)|/(dD/dλ) toward the shorter wavelength side fromthe wavelength λ₀+Δλ₀ as described hereinabove, and in order to allowtransmission of all of the four waves and allow the zero-dispersionwavelength deviation Δλ₀ to be set as great as possible, the wavelength(λ₀+Δλ₀)−Δλ_(SPM-GVD) and the wavelength λ₄ of the channel 4 are set soas to coincide with each other. From those requirements, the values ofΔλ_(SPM-GVD), Δλ₀ and λ₀ are defined in the equations given below:

Δλ_(SPM-GVD)=|D_(allow)|/(dD/dλ)=1(ps/(nm·km))/0.08(ps/nm²·km))=12.5 nm

Δλ₀=(Δλ_(SPM-GVD)−Δλ_(WDM)−Δλ_(g))/2=1.75 nm

λ₀=λ₁+Δλ₀+Δλ_(g)=1,564.75 nm

The values given above are values obtained when the deviation Δλ₀ is inthe minimum. It is to be noted that, as the dispersion slope dD/dλdecreases, Δλ_(SPM-GVD) increases, which allows an increase of thedeviation Δλ₀.

While the case wherein no optical dispersion compensator is employed hasbeen described with reference to FIG. 13, an alternative case whereinsignal light arrangement of different channels is performed using anoptical dispersion compensator will be described subsequently. Inparticular, the optical wavelength multiplex transmission method of theseventh embodiment of the present invention can arrange signal lightwaves of different channels in such a manner, for example, asillustrated in FIG. 14 using an optical dispersion compensator. It is tobe noted that description is given also here of the case wherein signallight waves of four channels are wavelength multiplexed and transmitted.

In particular, signal light waves of four channels are first arrangedoutside a transmissible band Δλ_(SPM-GVD) defined by an allowabledispersion value D_(allow) determined from an SPM-GVD effect in theoptical fiber 2 as illustrated at an upper half of FIG. 14, and then thezero-dispersion wavelength λ₀ of the optical fiber 2 is shifted to λ₀′as illustrated at a lower half of FIG. 14 using an optical dispersioncompensator to arrange the signal light waves of the four channelsapparently in the transmissible band Δλ_(SPM-GVD).

In this instance, the signal light waves of the four channels arearranged, before they are shifted by the optical dispersion compensator,at the equal spacing Δλ_(S) on the shorter wavelength side than thewavelength λ₀−Δλ₀−Δλ_(g) and within the gain bandwidth Δλ_(EDFA) of theEDFA similarly as in the example of an arrangement described hereinabovewith reference to FIG. 13. It is to be noted that the wavelength λ₁ ofthe channel 1 is set so that it may coincide with the wavelengthλ₀−Δλ₀−Δλ_(g) displaced by Δλ₀+Δλ_(g) toward the shorter wavelength sidefrom the zero-dispersion wavelength λ₀.

Then, by shifting the actual zero-dispersion wavelength λ₀ by Δλ_(DC)toward the shorter wavelength side by means of the optical dispersioncompensator, the signal light waves of the four channels are arrangedapparently in the transmissible bandwidth Δλ_(SPM-GVD) as seen in thelower half of FIG. 14.

It is to be noted that, though not illustrated in FIG. 14, when theproductivity of semiconductor lasers (light sources of signal lightwaves) and/or the optical wavelength variations of signal light wavescaused by the wavelength control accuracy are taken into consideration,the bandwidth Δλ_(WDM) within which signal light waves of a plurality ofchannels are to be arranged is set in an expanded condition inaccordance with such variations.

Further, though not illustrated in FIG. 14, where an optical dispersioncompensator is employed as described above, taking the dispersioncompensation amount deviation range ±δλ_(DC) of the optical dispersioncompensator into consideration, the signal light bandwidth Δλ_(WDM) isset expanding the same by the dispersion compensation amount deviationrange δλ_(DC) on the opposite sides of the longer wavelength side andthe shorter wavelength side. Further, for the optical dispersioncompensator, such optical dispersion compensators, for example, ashereinafter described in connection with ninth to fifteenth embodimentsof the present invention can be employed.

Here, the example of a signal light arrangement illustrated in FIG. 14is described using an example of detailed values. It is to be noted thatthe case wherein the zero-dispersion wavelength deviation Δλ₀ of theoptical fiber 2 can be set to the maximum using an optical dispersioncompensator having a dispersion value of the opposite positive ornegative sign to that of the transmission line of a signal band and thedispersion compensation wavelength shift amount Δλ_(DC) can be minimizedfrom the points of the size and the optical loss of the opticaldispersion compensator is considered here. Further, as regards numericalvalues, it is assumed here that they are similar to those describedhereinabove with reference to FIG. 13.

The zero-dispersion wavelength deviation Δλ₀ is allowed to be set to themaximum when an area over which the range of Δλ_(SPM-GVD) toward thelonger wavelength side from the lower limit of the zero-dispersionwavelength deviation and the range of Δλ_(SPM-GVD) toward the shorterwavelength side from the lower limit of the zero-dispersion wavelengthdeviation overlap with each other coincides with the signal lightbandwidth Δλ_(WDM) as seen from the lower half of FIG. 14. In short,

Δλ0(max)=(2·Δλ_(SPM-GVD)−Δλ_(WDM))/2=(2×12.5−6)/2=9.5 nm

and in this instance, the apparent zero-dispersion wavelength λ₀′ afterdispersion compensation is positioned at the center of the signal lightbandwidth Δλ_(WDM).

Before dispersion compensation, as shown in the upper half of FIG. 14,the wavelength λ₁ of the channel 1 is displaced by Δλ₀+Δλ_(g) toward theshorter wavelength side from the zero-dispersion wavelength λ₀ from therequirement for FWM suppression. Accordingly,

λ₀=λ₁+Δλ₀+Δλ_(g)=1,572.5 nm

and accordingly, λ₀±Δλ₀=1,572.5±9.5 nm.

In this instance, the dispersion compensation wavelength shift amountΔλ_(DC) is λ₀−λ₀′, and is calculated in the following manner:

Δλ_(DC)=(2·Δλ₀−Δλ_(SPM-GVD))+Δλ_(g)+Δλ_(WDM)=(2×9.5−12.5)+3+6=15.5 nm

Optical dispersion compensators are required to have a higherdispersion, a lower loss and a smaller size, and various types ofoptical dispersion compensators including the dispersion compensationfiber type, the transversal filter type and the optical resonator typehave been proposed. Here, optical dispersion compensators of the opticaldispersion compensation type, which will be hereinafter described inconnection with the ninth to fifteenth embodiments of the presentinvention, are employed.

It is to be noted that, since the example illustrated in FIG. 14requires an optical dispersion compensator having a positive dispersionvalue, if, for example, an ordinary single mode fiber (dispersion valueD_(DC)=18 ps/(nm·km)) is employed, then the required fiber length L_(DC)is given in the following manner:

 L_(DC)=(Δλ_(DC)·dD/dλ·L_(R-rep))/D_(DC)=(15.5×0.08×280)/18=19.3 km

While, in the examples of FIGS. 13 and 14 described hereinabove,detailed examples have been described for the cases wherein thezero-dispersion wavelength deviation Δλ₀ has the minimum value and themaximum value, respectively, the relationship of the zero-dispersionwavelength λ₀ and the dispersion compensation wavelength shift amountΔλ_(DC) to the deviation Δλ₀ where signal light waves are arranged onthe shorter wavelength side than the zero-dispersion wavelength λ₀ isillustrated in FIG. 22. In FIG. 22, the relationship where thewavelength spacing Δλ_(S) is Δλ₀=2 nm and the guard band Δλ_(g) isΔλ_(g)=3 nm is indicated by a solid line. Meanwhile, the relationshipwhere the wavelength spacing Δλ_(S) is Δλ_(S)=3 nm is indicated by abroken line. In this instance, from FIG. 13, since it is only requiredthat the zero-dispersion wavelength λ₀ and the wavelength λ₁ of thechannel 1 do not coincide with each other, the guard band Δλ_(g) is setto Δλ_(g)=1 nm.

In this manner, with the optical wavelength multiplex transmissionmethod of the seventh embodiment, signal light waves of differentchannels can be arranged without being influenced by FWM in an opticalamplifier multi-repeater WDM method which makes use of a band in theproximity of the zero-dispersion wavelength λ₀ of the optical fiber 2,and simultaneously, a required characteristic regarding thezero-dispersion wavelength λ₀ of an optical fiber transmission line tobe laid can be made definite and a channel arrangement method for signallight and a transmission line designing method in an optical amplifiermulti-repeater WDM method can be established.

Particularly, according to the present embodiment, by arranging signallight waves of different channels on the shorter wavelength side thanthe wavelength λ₀−Δλ₀−Δλ_(g) taking the zero-dispersion wavelengthdeviation range and the FWM suppressing guard band into consideration,the zero-dispersion wavelength deviation in the longitudinal directionof the optical fiber 2 is taken into consideration and controlled, andsimultaneously, an influence of FWM is suppressed. Consequently, aninfluence from another channel by crosstalk is suppressed, and a highdegree of transmission accuracy can be maintained.

Further, according to the present invention, in addition to the factthat signal light arrangement can be performed taking waveformdegradation by an SPM-GVD effect into consideration, the powers of thesignal light waves can be made equal and the received characteristicsfor the signal light waves can be made equal by arranging the signallight waves of the different channels within the gain bandwidthΔλ_(EDPA) of the EDFA.

Furthermore, by setting the bandwidth Δλ_(WDM), within which signallight waves are to be arranged, in an expanded condition in accordancewith optical wavelength variations of the signal light waves of thedifferent channels, the variations of the signal light waves caused bythe productivity and/or the wavelength control accuracy of light sourcesfor the signal light waves such as semiconductor lasers are taken intoconsideration, and where an optical dispersion compensator is employed,by setting the bandwidth Δλ_(WDM), within which the signal light wavesare to be arranged, expanding the same by the dispersion compensationamount deviation range δλ_(DC) of the optical dispersion compensator onthe opposite sides of the longer wavelength side and the shorterwavelength side, also the dispersion compensation amount deviation ofthe optical dispersion compensator is taken into consideration.Consequently, optical transmission of higher reliability can beachieved.

It is to be noted that, while, in the seventh embodiment describedabove, the case wherein signal light waves of four channels are to bearranged is described above, the present invention is not limited tothis.

H. Eighth Embodiment

Subsequently, an optical wavelength multiplex transmission method of aneighth preferred embodiment of the present invention. FIG. 23illustrates a signal light arrangement of a plurality of channels of theoptical wavelength multiplex transmission method while FIG. 24illustrates a modification to the signal light arrangement illustratedin FIG. 23, and FIG. 25 illustrates the relationship of thezero-dispersion wavelength and the dispersion compensation amount to thezero-dispersion wavelength deviation in the optical wavelength multiplextransmission method. It is to be noted that also the optical wavelengthmultiplex transmission method of the eighth embodiment is applied to asystem similar to the regenerative-repeater system or opticaltransmission system described hereinabove with reference to FIG. 15, andoverlapping description of the same will be omitted herein to avoidredundancy.

While, in the seventh embodiment described above, description has beengiven of the case wherein signal light waves of different channels arearranged on the shorter wavelength side than the zero-dispersionwavelength λ₀ of the optical fiber 2, in the eighth embodiment, signallight waves of different channels are arranged on the longer wavelengthside than the zero-dispersion wavelength λ₀ of the optical fiber 2.Then, after the wavelength λ₁ of the channel 1 is set to the shorterwavelength end 1,550 nm of the gain bandwidth of the EDFA, therelationship between the channel arrangement and characteristicsrequired for the DSF (optical fiber 2) are determined by the quite samemeans as that of the seventh embodiment described hereinabove withreference to FIG. 13.

In particular, taking the zero-dispersion wavelength λ₀ of the opticalfiber 2 and the zero-dispersion wavelength deviation ±Δλ₀ in thelongitudinal direction of the optical fiber 2 into consideration, signallight waves of fourth channels to be multiplexed are arranged at anequal spacing Δλ_(S) on the longer wavelength side than the longerwavelength end λ₀+Δλ₀ of the zero-dispersion wavelength deviation rangeof the optical fiber 2 as illustrated in FIG. 23.

In this instance, an FWM suppressing guard band Δλ_(g) is provided onthe longer wavelength side than the longer wavelength end λ₀+Δλ₀ of thezero-dispersion wavelength deviation range of the optical fiber 2, andthe signal light waves of the four channels (for the channels 1 to 4 ofthe wavelengths λ₁ to λ₄) are arranged on the further longer wavelengthside than the wavelength λ₀+Δλ₀+Δλ_(g). In the present embodiment, thewavelength λ₁ of the channel 1 is set at the position displaced byΔλ₀+Δλ_(g) toward the longer wavelength side from the zero-dispersionwavelength λ₀ of the DSF (optical fiber 2), that is, the wavelengthλ₀+Δλ₀+Δλ_(g) is set so as to coincide with the wavelength λ₁ of thechannel 1.

Further, in the present embodiment, signal light waves of four channelsare arranged in a transmissible band Δλ_(SPM-GVD) defined by anallowable dispersion value D_(allow) determined from an SPM-GVD in theoptical fiber 2. In particular, as illustrated in FIG. 23, thetransmissible signal light wavelength range is a range withinΔλ_(SPM-GVD)=|D_(allow)|/(dD/dλ) displaced toward the longer wavelengthside from the shorter wavelength end λ₀−Δλ₀ of the zero-dispersionwavelength deviation range of the optical fiber 2. In this instance, inorder to allow the four waves to be transmitted and allow thezero-dispersion wavelength deviation Δλ₀ to be set as great as possible,the wavelength λ_(SPM-GVD) (=(λ₀−Δλ₀)+Δλ_(SPM-GVD)) and the wavelengthλ₄ of the channel 4 are set so as to coincide with each other.

Further, in the present embodiment, the signal light waves of the fourchannels are arranged within a gain bandwidth Δλ_(EDFA) (for example,such a range of 1,550 to 1,560 nm as shown in FIG. 16) of an EDFAconnected to the optical fiber 2.

It is to be noted that, though not illustrated in FIG. 23, also in thepresent embodiment, when the productivity of semiconductor lasers (lightsources of signal light waves) and/or the optical wavelength variationsof the signal light waves caused by the wavelength control accuracy aretaken into consideration, the bandwidth Δλ_(WDM) within which signallight waves of a plurality of channels are to be arranged is set in anexpanded condition in accordance with such variations.

By the way, while the case wherein an optical dispersion compensator isnot employed is described above with reference to FIG. 23, another casewherein signal light arrangement of different channels is performedusing an optical dispersion compensator will be described subsequently.In other words, with the optical wavelength multiplex transmissionmethod of the eighth embodiment of the present invention, signal lightwaves of different channels can be arranged, for example, in such amanner as illustrated in FIG. 24 by using an optical dispersioncompensator.

In particular, signal light waves of four channels are first arrangedoutside a transmissible band Δλ_(SPM-GVD) defined by an allowabledispersion value Dallow determined by an SPM-GVD effect in the opticalfiber 2 as illustrated in the upper half of FIG. 24, and then thezero-dispersion wavelength λ₀ of the optical fiber 2 is shifted to λ₀′using an optical dispersion compensator as illustrated in the lower halfof FIG. 24 to arrange the signal light waves of the four channelsapparently within the transmissible band Δλ_(SPM-GVD).

In this instance, the signal light waves of the four channels arearranged, before shifting by the optical dispersion compensator isperformed, at an equal spacing Δλ_(S) on the longer wavelength side thanthe wavelength λ₀+Δλ₀+Δλ_(g) and within the gain bandwidth Δλ_(EDFA) ofthe EDFA similarly as in the example of an arrangement describedhereinabove with reference to FIG. 23. It is to be noted that thewavelength λ₁ of the channel 1 is set so as to coincide with thewavelength λ₀+Δλ₀+Δλ_(g) displaced by Δλ₀+Δλ_(g) toward the longerwavelength side from the zero-dispersion wavelength λ₀.

Then, the actual zero-dispersion wavelength λ₀ is shifted by Δλ₀(=λ₀′−λ₀) toward the longer wavelength side by means of the opticaldispersion compensator thereby to apparently arrange the signal lightwaves of the four channels within the transmissible band Δλ_(SPM-GVD).

It is to be noted that also FIG. 24 illustrates the case wherein, asdescribed hereinabove in connection with the seventh embodiment withreference to FIG. 14, an area over which the range of Δλ_(SPM-GVD)displaced toward the longer wavelength side from the lower limit of thezero-dispersion wavelength deviation and the range of Δλ_(SPM-GVD)displaced toward the shorter wavelength side from the lower limit of thezero-dispersion wavelength deviation overlap with each other is madecoincide with the signal light bandwidth Δλ_(WDM) so that thezero-dispersion wavelength deviation Δλ₀ is allowed to be set to themaximum as described hereinabove in connection with the seventhembodiment with reference to FIG. 14.

Further, though not illustrated in FIG. 24, when the productivity ofsemiconductor lasers (light sources of the signal light waves) and/orthe optical wavelength variations of the signal light waves caused bythe wavelength control accuracy are taken into consideration, thebandwidth Δλ_(WDM) within which signal light waves of a plurality ofchannels are to be arranged is set in an expanded condition inaccordance with such variations.

Further, though not illustrated in FIG. 24, where an optical dispersioncompensator is employed as described above, taking the dispersioncompensation amount deviation range ±δλ_(DC) of the optical dispersioncompensator into consideration, the signal light bandwidth Δλ_(WDM) isset expanding the same by the dispersion compensation amount deviationrange δλ_(DC) on the opposite sides of the longer wavelength side andthe shorter wavelength side. Further, for the optical dispersioncompensator, such optical dispersion compensators, for example, ashereinafter described in connection with ninth to fifteenth embodimentsof the present invention can be employed.

While, in the examples of FIGS. 23 and 24 described hereinabove, thecases wherein the zero-dispersion wavelength deviation Δλ₀ has theminimum value and the maximum value, respectively, have been described,the relationship of the zero-dispersion wavelength λ₀ and the dispersioncompensation wavelength shift amount Δλ_(DC) to the deviation Δλ₀ wheresignal light waves are arranged on the longer wavelength side than thezero-dispersion wavelength λ₀ is illustrated in FIG. 25. Also in FIG.25, similar numerical values to those described hereinabove inconnection with the seventh embodiment with reference to FIG. 22 areapplied. However, in FIG. 25, the slope of the zero-dispersionwavelength λ₀ relative to the deviation Δλ₀ is set opposite to thatillustrated in FIG. 22 in order to arrange the signal light waves on thelonger wavelength side than the zero-dispersion wavelength λ₀.

In this manner, similar advantages to those described hereinabove inconnection with the seventh embodiment can be achieved by the opticalwavelength multiplex transmission method of the eighth embodiment.

It is to be noted that, while, in the eighth embodiment described above,the case wherein the signal light waves of the four channels are to bearranged has been described, the present invention is not limited tothis, and the signal light waves of the channels can be arranged on theopposite sides of the zero-dispersion wavelength λ₀. In this instance,when optical dispersion compensation is involved, different opticaldispersion compensators of the opposite positive and negative signs mustnecessarily be used for the channels on the shorter wavelength side andthe longer wavelength side of the zero-dispersion wavelength λ₀.

I. Ninth Embodiment

Subsequently, an optical dispersion compensation method as a ninthpreferred embodiment of the present invention will be described. FIG. 26shows, in block diagram, an optical dispersion compensation system towhich the optical dispersion compensation method is applied. Referringto FIG. 26, the optical dispersion compensation system shown is denotedat 20 and includes a transmitter 21 for converting an electric signalinto an optical signal and transmitting the optical signal, and aplurality of repeaters 22 inserted in an optical transmission line(optical fiber 2). Such an in-line repeater or a regenerative-repeateras described hereinabove may be employed for the repeaters 22.

The optical dispersion compensation system 20 further includes areceiver 23 for converting a received optical signal into an electricsignal. The transmitter 21 and the receiver 23 are interconnected by wayof the optical fiber 2 with the repeaters 22 interposed in the opticalfiber 2. In the optical transmission system 20, signal light from thetransmitter 21 is transmitted to the receiver 23 by way of the repeaters22 and the optical fiber 2.

The optical dispersion compensation system 20 further includes two kindsof optical dispersion compensator units including an optical dispersioncompensator unit 24A having a positive dispersion amount +B and anotheroptical dispersion compensator unit 24B having a negative dispersionamount −B. The two kinds of optical dispersion compensator units 24A and24B are prepared in advance and are interposed in the opticaltransmission system 20, that is, at any location of the optical fiber 2,the transmitter 21, the repeaters 22 and the receiver 23.

By the way, where the optical transmission system 20 is such an opticalamplifier regenerative-repeater system as described hereinabove withreference to FIG. 15, since the allowable dispersion value decreases asthe regenerative-repeater span increases as described hereinabove withreference to FIG. 19, an optical dispersion compensator for restrainingthe arrangement positions of the channels (signal light) within anallowable dispersion range for the arrangement positions is essentiallyrequired.

Further, while, in the first to eighth embodiments describedhereinabove, the zero-dispersion wavelength of the optical fiber 2 andthe signal light wavelength are separated from each other in order toeliminate otherwise possible crosstalk by FWM in the WDM method whichmakes use of a band in the proximity of the zero-dispersion wavelengthof the optical fiber 2, dispersion compensation by the correspondingamount (refer particularly to the examples of FIGS. 14 and 24 in theseventh and eighth embodiments) is required. Such dispersioncompensation is required also for one-wave transmission or SMFtransmission.

Particularly in the case of an optical communication system on land,since the repeater span is not fixed and besides the zero-dispersionwavelength of an actual optical fiber exhibits a deviation in thelongitudinal direction, it is difficult to set the dispersion amounts ofdifferent repeater sections equal to each other. Therefore, when asignal light wavelength is set in the proximity of the zero-dispersionwavelength of the DSF (optical fiber 2), there is even the possibilitythat the positive or negative sign of the dispersion amount may bedifferent among different repeater sections.

Thus, in the present ninth embodiment, in order to compensate for thedispersion amount of the optical transmission system 20, the two kindsof optical dispersion compensator units 24A and 24B prepared in advanceare inserted into the optical transmission system 20, and one of theoptical dispersion compensator units 24A and 24B which provides a bettertransmission characteristic to the optical transmission system 20 isselected and incorporated into the optical transmission system 20.

Consequently, when an accurate dispersion amount cannot be measured andthe zero-dispersion wavelength deviation can be grasped to some degree,the dispersion amount of the optical transmission system 20 can becompensated for readily.

On the other hand, when the dispersion amount of the opticaltransmission system 20 can be measured, by selecting one of the opticaldispersion compensator units 24A and 24B which has the sign opposite tothe sign of the measured dispersion amount, the dispersion amount of theoptical transmission system 20 can be compensated for with a higherdegree of certainty.

In this manner, with the optical dispersion compensation method of theninth embodiment, the waveform degradation by an SPM-GVD effect or thedispersion amount of a guard band can be compensated for withoutdesigning or producing optical dispersion compensators conforming toindividual transmission lines, and reduction of the number of steps andreduction of the time until an optical communication system is built upcan be realized.

Here, an example of detailed numerical values of the ninth embodimentwill be described. If it is assumed that the transmission rate is 10Gbps; the in-line repeater span L_(in-line) is 70 km; the variation ofthe optical output of each optical amplifier is ±2 dB, from FIG. 19, themaximum regenerative-repeater span is 280 km at the allowable dispersionvalue D_(allow)±1 ps/(nm·km), and accordingly, the dispersioncompensation of ±280 ps/nm is required for the dispersion amount ofsignal light after transmission of 280 km. Therefore, where thetransmission line dispersion amount is, for example, +1,200 ps/nm, whenthe optical dispersion compensator units 24A and 24B of the dispersionamounts +1,000 ps/nm and −1,000 ps/nm are prepared, if the opticaldispersion compensator unit 24B of the dispersion amount −1,000 ps/nm isinserted into the transmission line, then the total dispersion amount is+200 ps/nm, and therefore, transmission is possible.

J. Tenth Embodiment

Subsequently, an optical dispersion compensation method of a tenthpreferred embodiment of the present invention will be described. FIG. 27shows, in block diagram, an optical dispersion compensation apparatus towhich the optical dispersion compensation method is applied. In FIG. 27,like elements are denoted by like reference characters to those of FIG.26, and overlapping description thereof is omitted herein to avoidredundancy.

While, in the ninth embodiment described above, the two kinds of opticaldispersion compensator units having the positive dispersion amount +Band the negative dispersion amount −B are prepared in advance, in thepresent tenth embodiment, a plurality of kinds of optical dispersioncompensators 25A and 25B having different dispersion amounts havingdifferent positive and negative signs are prepared in advance.

Here, two kinds of optical dispersion compensator units 25A and 25Bhaving dispersion amounts B1 and B2 are prepared each by a pluralnumber, and an optical dispersion compensator unit 25 which isconstituted from a combination of such optical dispersion compensationunits 25A and 25B is inserted into the optical transmission system 20,that is, at any portion of the optical fiber 2, the transmitter 21, therepeaters 22 and the receiver 23.

Further, in the present embodiment, at a cite at which an opticalcommunication system is to be installed, the two kinds of opticaldispersion compensator units 25A and 25B are inserted into the opticaltransmission system 20 changing the number and the combination of unitsto be installed, and the transmission characteristic, particularly thecode error rate, of the optical transmission system 20 is measured.Then, an optical dispersion compensator unit 25 of the number and thecombination of units which provide a good transmission characteristic(in FIG. 27, the combination of three optical dispersion compensatorunits 25A and one optical dispersion compensator unit 25B) isselectively determined from the two kinds of optical dispersioncompensator units 25A and 25B and incorporated into the opticaltransmission system 20.

Consequently, even when the zero-dispersion wavelength deviation is notknown or when the zero-dispersion and the signal light wavelength aredisplaced by a great amount from each other, the dispersion amount ofthe optical transmission system 20 can be compensated for readily andoptimally.

In contrast, when the dispersion amount of the optical transmissionsystem 20 can be measured, the dispersion amount is measured first, andthen an optical dispersion compensator unit 25 of the installationnumber and the combination of units with which the dispersion value ofsignal light falls within a transmissible dispersion value range isselectively determined from the two kinds of optical dispersioncompensator units 25A and 25B and incorporated into the opticaltransmission system 20. Consequently, the dispersion amount of theoptical transmission system 20 can be compensated for so that it can beaccommodated into the allowable dispersion value range with certainty.

In this manner, also with the optical dispersion compensation method ofthe tenth embodiment, the waveform degradation by an SPM-GVD effect orthe dispersion amount of a guard band can be compensated for withoutdesigning or producing optical dispersion compensators conforming toindividual transmission lines, and reduction of the number of steps andreduction of the time until an optical communication system is built upcan be realized.

It is to be noted that, while, in the tenth embodiment described above,description has been given of the case wherein two kinds of opticaldispersion compensator units are prepared in advance, the presentinvention is not limited to this.

Here, an example of detailed numerical values of the tenth embodimentwill be described. Where the dispersion compensation of ±280 ps/nm isrequired as a dispersion amount for signal light after transmission overthe distance of 280 km, if it is assumed that optical dispersioncompensator units having the dispersion amounts A1, A2, B1 and B2, forexample, of +300 ps/nm, +100 ps/nm, −300 ps/nm and −100 ps/nm,respectively, are prepared in advance, then if three optical dispersioncompensator units of the dispersion amount B1 and one optical dispersioncompensator unit of the dispersion amount B2 are inserted in combinationinto the transmission line, then the total dispersion amount is +200ps/nm, which allows transmission.

K. Eleventh Embodiment

Subsequently, an optical dispersion compensation method of an eleventhpreferred embodiment of the present invention will be described. FIG. 28shows, in block diagram, an optical dispersion compensation apparatus towhich the optical dispersion compensation method is applied, and FIGS.29 and 30 show different modifications to the optical dispersioncompensation apparatus. It is to be noted that, while, in the ninth andtenth embodiments described above, description has been given only oftransmission of one signal light wave, in the present embodiment,description will be given of the case wherein signal light waves(wavelengths λ₁ to λ₄) of four channels are wavelength multiplexed andtransmitted.

As seen from FIG. 28, also in the present embodiment, an opticaltransmission system 20 is constituted from a transmitter 21, a pluralityof repeaters 22 and a receiver 23 interconnected by an optical fiber 2.However, in the present eleventh embodiment, the transmitter 21 isconstructed so as to first convert electric signals of differentchannels into signal light waves having different wavelengths orfrequencies from one another and then multiplex the signal light wavesby optical wavelength multiplexing. To this end, the transmitter 21includes a plurality of electro-optical conversion sections (E/O1 toE/O4) 21 a provided for the individual channels for converting electricsignals of the channels into signal light waves of the predeterminedwavelengths, and an optical multiplexing section 21 b for receivingsignal light waves from the electro-optical conversion sections 21 a forthe individual channels and multiplexing the received signal lightwaves.

Meanwhile, the receiver 23 demultiplexes multiplexed signal lighttransmitted thereto from the transmitter 21 by way of the optical fiber2 and the repeaters 22 and converts signal light waves obtained by suchdemultiplexing individually into electric signals. To this end, thereceiver 23 includes an optical demultiplexing section 23 a fordemultiplexing and distributing multiplexed signal light into differentchannels, and a plurality of opto-electric conversion sections (O/E1 toO/E4) 23 b provided individually for the channels for converting signallight waves of the channels distributed thereto from the opticaldemultiplexing section 23 a into electric signals.

Further, in the present embodiment, optical dispersion compensator units25 are interposed between the electro-optical conversion sections 21 aand the optical multiplexing section 21 b of the transmitter 21. Inparticular, a suitable number and combination of optical dispersioncompensator units 25A and 25B are provided for each of signal lightwaves of wavelengths λ₁ to λ₄ before wavelength multiplexing.

In the arrangement shown in FIG. 28, for the channel of the wavelengthλ₁, only one optical dispersion compensator unit 25A of the dispersionamount B1 is provided; for the channel of the wavelength λ₂₁ one opticaldispersion compensator unit 25A of the dispersion amount B1 and oneoptical dispersion compensator unit 25B of the dispersion amount B2 areprovided; for the channel of the wavelength λ₃, one optical dispersioncompensator unit 25A of the dispersion amount B1 and two opticaldispersion compensator units 25B of the dispersion amount B2 areprovided; and for the channel of the wavelength λ₄, one opticaldispersion compensator unit 25A of the dispersion amount B1 and threeoptical dispersion compensator units 25B of the dispersion amount B2 areprovided.

In this instance, when the installation number and the combination ofthe optical dispersion compensator units 25A and 25B arranged for thedifferent channels are to be selected, as described hereinabove in theninth and tenth embodiments, those which provide good transmissioncharacteristics for the individual channels may be selected by trial anderror or, when the dispersion value of the optical transmission system20 can be measured, those with which the dispersion values of signallight waves fall within transmissible dispersion value ranges may beselected in accordance with a result of the measurement.

While the arrangement wherein the optical dispersion compensator units25 are provided in the transmitter 21 are shown in FIG. 28, such opticaldispersion compensator units 25 may be provided alternatively in eachrepeater 22 or the receiver 23 as seen in FIG. 29 or 30.

As shown in FIG. 29, where the optical dispersion compensator units 25are provided in each repeater 22, the repeater 22 includes, in additionto an optical amplifier 22 a constituting the repeater 22, an opticaldemultiplexing section 22 b provided at a next stage to the opticalamplifier 22 a for demultiplexing signal light amplified by the opticalamplifier 22 a into individual signal light waves of differentwavelengths λ₁ to λ₄ by wavelength demultiplexing, an optical dispersioncompensator unit 25 provided for each of the channels of signal lightwaves of the wavelengths λ₁ to λ₄ demultiplexed by the opticaldemultiplexing section 22 b and including a suitable installation numberand a suitable combination of optical dispersion compensator units 25Aand 25B, and an optical multiplexing section 22 c for multiplexingsignal light waves of the channels dispersion compensated for by theoptical dispersion compensator units 25 back into signal light bywavelength multiplexing and sending out the thus multiplexed signallight into a transmission line. It is to be noted that the opticaldemultiplexing section 22 b, the optical dispersion compensator units 25and the optical multiplexing section 22 c may be provided otherwise at apreceding stage to the optical amplifier 22 a.

On the other hand, where the optical dispersion compensator units 25 areto be provided in the receiver 23, as shown in FIG. 30, they areinterposed between the optical demultiplexing section 23 a and theopto-electric conversion sections 23 b of the receiver 23. Inparticular, a suitable installation number and a suitable combination ofoptical dispersion compensator units 25A and 25B are provided for eachof the signal light waves of the wavelengths λ₁ to λ₄ after wavelengthdemultiplexing.

In this manner, with the optical dispersion compensation method of theeleventh embodiment, also where the optical transmission system 20performs optical wavelength multiplex transmission to multiplex andtransmit signal light waves of different wavelengths, similar advantagesto those described hereinabove in connection with the ninth and tenthembodiments can be attained by providing a suitable installation numberand a suitable combination of optical dispersion compensator units 25Aand 25B for each wavelength.

It is to be noted that, while the embodiment described above involvesfour channels of signal light waves to be multiplexed and two kinds ofoptical dispersion compensator units prepared in advance for dispersioncompensation for the individual channels, the present invention is notlimited to this.

L. Twelfth Embodiment

Subsequently, an optical dispersion compensation method of a twelfthpreferred embodiment of the present invention will be described. FIG. 31shows, in block diagram, an optical dispersion compensation apparatus towhich the optical dispersion compensation method is applied, and FIGS.32 and 33 show different modifications to the optical dispersioncompensation apparatus. It is to be noted that like reference charactersdenote like elements to those described hereinabove, and overlappingdescription thereof is omitted herein to avoid redundancy.

While, in the eleventh embodiment described above, description has beengiven of the case wherein a suitable installation number and a suitablecombination of optical dispersion compensator units 25A and 25B areprovided for each wavelength, in the present twelfth embodiment, asuitable installation number and a suitable combination of opticaldispersion compensator units 25A and 25B are provided in the opticaltransmission system 20 for each channel group including a plurality ofsignal light waves (two signal light waves in the present embodiment).

In particular, FIGS. 31 to 33 illustrate different arrangements whereinoptical dispersion compensator units 25 are provided in the transmitter21, each of the repeaters 22 and the receiver 23, respectively. Wherethe optical dispersion compensator units 25 are provided in thetransmitter 21 as shown in FIG. 31, the optical multiplexing section 21b of the transmitter 21 described hereinabove includes an opticalmultiplexing section 21 c for multiplexing signal light waves of thewavelengths λ₁ and λ₂ from the electro-optical conversion section 21 a,another optical multiplexing section 21 d for multiplexing signal lightwaves of the wavelengths λ₃ and λ₄ from the electro-optical conversionsection 21 a, and a further optical multiplexing section 21 e formultiplexing two signal light beams multiplexed by the opticalmultiplexing sections 21 c and 21 d.

An optical dispersion compensator unit 25 is interposed between each ofthe multiplexing sections 21 c and 21 d and the optical multiplexingsection 21 e. In other words, a suitable installation number and asuitable combination of optical dispersion compensator units 25A and 25Bare provided for each of channel groups each including two signal lightwaves.

For example, in the arrangement shown in FIG. 31, for the channel groupof the wavelengths λ₁ and λ₂, only one optical dispersion compensatorunit 25A of the dispersion amount Bi is provided; and for the channelgroup of the wavelengths λ₃ and λ₄, one optical dispersion compensatorunit 25A of the dispersion amount B1 and one optical dispersioncompensator unit 25B of the dispersion amount B2 are provided.

In this instance, when the installation number and the combination ofthe optical dispersion compensator units 25A and 25B to be arranged forthe different channels are to be selected, as described hereinabove inthe ninth and tenth embodiments, those which provide good transmissioncharacteristics for the individual channels may be selected by trial anderror or, when the dispersion amount of the optical transmission system20 can be measured, those with which the dispersion values of signallight waves fall within a transmissible dispersion value range may beselected in accordance with a result of the measurement.

Meanwhile, where the optical dispersion compensator units 25 areprovided in each repeater 22 as shown in FIG. 32, the repeater 22includes, in addition to the optical amplifier 22 a constituting therepeater 22, an optical demultiplexing section 22 d provided at a nextstage to the optical amplifier 22 a for demultiplexing signal lightamplified by the optical amplifier 22 a into two channel groupsincluding a group of the wavelengths λ₁ and λ₂ and another group of thewavelengths λ₃ and λ₄ by wavelength demultiplexing, an opticaldispersion compensator unit 25 provided for each of the channel groupsdemultiplexed by the optical demultiplexing section 22 d and including asuitable installation number and a suitable combination of opticaldispersion compensator units 25A and 25B, and an optical multiplexingsection 22 e for multiplexing signal light waves of the channel groupsdispersion compensated for by the optical dispersion compensator units25 back into signal light by wavelength multiplexing and sending out thethus multiplexed signal light into the transmission line. It is to benoted that the optical demultiplexing section 22 d, the opticaldispersion compensator units 25 and the optical multiplexing section 22e may be provided otherwise at a preceding stage to the opticalamplifier 22 a.

On the other hand, where the optical dispersion compensator units 25 areto be provided in the receiver 23, as shown in FIG. 33, the opticaldemultiplexing section 23 a of the receiver 23 described above includesan optical demultiplexer 23 c for demultiplexing received signal lightinto a channel group of the wavelengths λ₁ and λ₂ and another channelgroup of the wavelengths λ₃ and λ₄, another optical demultiplexingsection 23 d for demultiplexing the channel group of the wavelengths λ₁and λ₂ into signal light waves of the wavelengths λ₁ and λ₂, and afurther optical demultiplexing section 23 e for demultiplexing thechannel group of the wavelengths λ₃ and λ₄ into signal light waves ofthe wavelengths λ₃ and λ₄.

Further, an optical dispersion compensator unit 25 is interposed betweenthe optical demultiplexing section 23 c and each of the opticaldemultiplexing sections 23 d and 23 e. In particular, a suitableinstallation number and a suitable combination of optical dispersioncompensator units 25A and 25B are provided for each of the channelgroups each including two signal light waves.

In this manner, with the optical dispersion compensation method of thetwelfth embodiment, also where the optical transmission system 20performs optical wavelength multiplex transmission to multiplex andtransmit signal light waves of different wavelengths, similar advantagesto those described hereinabove in connection with the ninth and tenthembodiments can be attained by providing a suitable installation numberand a suitable combination of optical dispersion compensator units 25Aand 25B for each channel group.

It is to be noted that, while the embodiment described above involvesfour channels of signal light waves to be multiplexed and two kinds ofoptical dispersion compensator units prepared in advance for dispersioncompensation for the individual channels and besides involves separationof the channels into two channel groups, the present invention is notlimited to this.

M. Thirteenth Embodiment

Subsequently, an optical dispersion compensation method of a thirteenthpreferred embodiment of the present invention will be described. FIG. 34shows, in block diagram, an optical dispersion compensation apparatus towhich the optical dispersion compensation method is applied, and FIGS.35 and 36 show different modifications to the optical dispersioncompensation apparatus. It is to be noted that like reference charactersdenote like elements to those described hereinabove, and overlappingdescription thereof is omitted herein to avoid redundancy.

While, in the eleventh or twelfth embodiment described above,description has been given of the case wherein a suitable installationnumber and a suitable combination of optical dispersion compensatorunits 25A and 25B are provided for each wavelength or for each channelgroup, in the present thirteenth embodiment, a suitable installationnumber and a suitable combination of optical dispersion compensatorunits 25A and 25B are provided in the optical transmission system 20 forall of signal light waves of a plurality of channels (four channels inthe arrangement shown in FIG. 34).

In particular, FIGS. 34 to 36 illustrate different arrangements whereinan optical dispersion compensator unit 25 is provided in the transmitter21, each of the repeaters 22 and the receiver 23, respectively. Wherethe optical dispersion compensator unit 25 is provided in thetransmitter 21 as shown in FIG. 34, the optical dispersion compensatorunit 25 is provided at a next stage to the optical multiplexing section21 b of the transmitter 21 and includes a suitable installation numberand a suitable combination of optical dispersion compensator units 25Aand 25B. For example, in the arrangement shown in FIG. 34, one opticaldispersion compensator unit 25A of the dispersion amount B1 and oneoptical dispersion compensator unit 25B of the dispersion amount B2 areprovided.

In this instance, when the installation number and the combination ofthe optical dispersion compensator units 25A and 25B to be arranged forall of the signal light waves are to be selected, as describedhereinabove in the ninth and tenth embodiments, those which provide goodtransmission characteristics for the individual channels may be selectedby trial and error or, when the dispersion amount of the opticaltransmission system 20 can be measured, those with which the dispersionvalues of signal light waves fall within a transmissible dispersionvalue range may be selected in accordance with a result of themeasurement.

Meanwhile, where the optical dispersion compensator unit 25 is providedin each repeater 22 as shown in FIG. 35, it is located at a next stageto the optical amplifier 22 a constituting the repeater 22 and includesa suitable installation number and a suitable combination of opticaldispersion compensator units 25A and 25B. It is to be noted that theoptical dispersion compensator unit 25 may be provided otherwise at apreceding stage to the optical amplifier 22 a.

On the other hand, where the optical dispersion compensator unit 25 isto be provided in the receiver 23, as shown in FIG. 36, it is located ata preceding stage to the optical demultiplexing section 23 a of thereceiver 23 and includes a suitable installation number and a suitablecombination of optical dispersion compensator units 25A and 25B.

In this manner, with the optical dispersion compensation method of thethirteenth embodiment, also where the optical transmission system 20performs optical wavelength multiplex transmission to multiplex andtransmit signal light waves of different wavelengths, similar advantagesto those described hereinabove in connection with the ninth and tenthembodiments can be attained by providing a suitable installation numberand a suitable combination of optical dispersion compensator units 25Aand 25B for all of signal light waves of the channels.

It is to be noted that, while the embodiment described above involvesfour channels of signal light waves to be multiplexed and two kinds ofoptical dispersion compensator units prepared in advance for dispersioncompensation for the individual channels, the present invention is notlimited to this.

Further, in the tenth to thirteenth embodiments described above, it isimportant to design the dispersion values of the involved opticaldispersion compensator units taking the wavelength spacing between thechannels and the dispersion slope dD/dλ of the transmission line intoconsideration and reduce the number of types of optical dispersioncompensator units as small as possible.

N. Fourteenth Embodiment

Subsequently, an optical dispersion compensation method of a fourteenthpreferred embodiment of the present invention will be described. FIG. 37shows, in block diagram, an optical dispersion compensation apparatus towhich the optical dispersion compensation method is applied, and FIGS.38(a) and 38(b) show a modification to the optical dispersioncompensation apparatus while FIG. 39 show another modification to theoptical dispersion compensation apparatus and FIG. 40 shows an exampleof the construction of a packet based on the modified optical dispersioncompensation apparatus of FIG. 39. It is to be noted that like referencecharacters denote like elements to those described hereinabove, andoverlapping description thereof is omitted herein to avoid redundancy.

While, in the ninth to thirteenth embodiments described above,description has been given of the arrangement means for the opticaldispersion compensator units 24A, 24B, 25, 25A and 25B, in the presentfourteenth embodiment, examples of a detailed construction and insertioninstallation means of the optical dispersion compensator units 25, 25Aand 25B themselves will be described.

For example, as shown in FIG. 37, an optical amplifier 26 isadditionally provided at a preceding stage or a next stage to each ofoptical dispersion compensator units 25A and 25B constituting an opticaldispersion compensator unit 25 for compensating the optical loss by theoptical dispersion compensator unit 25A or 25B.

By the way, various types of optical dispersion compensators have beenproposed so far including the dispersion compensating fiber type, thetransversal filter type and the optical resonator type. While dispersioncompensation fibers having a dispersion value higher than −100ps/(nm·km) are manufactured at present by contriving the shape of thecore, with such dispersion compensation fibers, the optical loss is highalthough a dispersion compensation amount can be adjusted readily by thelength of the fiber.

Thus, where the optical dispersion compensator units 25A and 25B areintegrated with an optical amplifier 26 such as an EDFA as in thefourteenth embodiment, the optical loss of the dispersion compensationfiber can be compensated for.

It is to be noted that, while an optical amplifier 26 is additionallyprovided for each optical dispersion compensator unit 25A or 25B in FIG.37, only one optical amplifier 26 may otherwise be provided for eachgroup (optical dispersion compensation unit 25) of optical dispersioncompensator units 25A and 25B as shown in FIG. 38(a) or 38(b).

Alternatively, a pair of optical amplifiers 26A and 26B are additionallyprovided at both of a preceding stage and a next stage to each group(optical dispersion compensator unit 25) of optical dispersioncompensator units 25A and 25B as shown in FIG. 39.

Where only one amplifier is provided, not only a high gain sufficient tocompensate for both of the transmission line loss and the optical lossat the optical dispersion compensator unit 25 is required, but where theoptical dispersion compensator unit 25 having a high optical loss islocated at a preceding stage to the optical amplifier 26, this makes acause to degrade the NF significantly. This must be eliminatedparticularly where an optical dispersion compensator unit 25 is insertedin a 1R repeater in an optical amplifier multi-repeater system.

Therefore, where such a construction as shown in FIG. 39 wherein the twooptical amplifiers 26A and 26B are provided on the opposite front andrear ends of the optical dispersion compensator unit 25 is employed, theNF of the entire 1R repeater can be reduced low by minimizing the NF ofthe optical amplifier at the preceding stage, and a sufficient gain canbe assured by means of the two stages of optical amplifiers 26A and 26B.

Incorporation of such an optical dispersion compensator unit 25 asdescribed above into the transmitter 21, each of the repeaters 22 or thereceiver 23 is performed, for example, in the following manner. A spacesufficient to allow insertion of an optical dispersion compensator unit25 therein is assured in advance in each of the transmitter 21, therepeaters 22 and the receiver 23, and after installation of the system,optimum optical dispersion compensator units 25 conforming to thetransmission line (optical transmission system 20) are additionallyinserted into the spaces to incorporate the optical dispersioncompensators 25 into the optical transmission system 20.

Meanwhile, electronic parts and optical parts in an optical transmissionapparatus are generally mounted on a printed circuit board (a printedcircuit board on which electronic parts and/or optical parts are mountedin this manner is called package), and such package in most cases has astructure which allows mounting and dismounting onto and from anapparatus support frame.

Thus, a dispersion compensation package having optical dispersioncompensator units mounted thereon may be provided so that it may bemounted and dismounted onto and from an apparatus supporting frame. Forexample, a package obtained by packaging the optical dispersioncompensator unit 25 shown in FIG. 39 is shown in FIG. 40. Referring toFIG. 40, an optical dispersion compensator unit 25 including a pair offront and rear optical amplifiers 26A and 26B and three opticaldispersion compensator units 25A and 25B of two different types ismounted on a printed circuit board 27 to constitute a dispersioncompensation package 28. It is to be noted that each of the opticaldispersion compensator units 25A and 25B is constituted from adispersion compensation fiber (optical fiber 2) wound by a predeterminedlength around a small bobbin located on the printed circuit board 27.

Where such a dispersion compensation package 28 as described above isemployed, optical dispersion compensator units 25 can be replaced orincorporated readily in units cf a package. Consequently, the dispersioncompensation amount can be varied readily.

O. Fifteenth Embodiment

Subsequently, an optical dispersion compensation method of a fifteenthpreferred embodiment of the present invention will be described. FIG. 41shows, in block diagram, an optical dispersion compensation apparatus towhich the optical dispersion compensation method is applied, and FIGS.42 and 43 show different modifications to the optical dispersioncompensation apparatus. It is to be noted that like reference charactersdenote like elements to those described hereinabove, and overlappingdescription thereof is omitted herein to avoid redundancy.

In the fifteenth embodiment, such an optical dispersion compensator unit32 is built in each of the transmitter 21, the repeaters 22 and thereceiver 23 which constitute the optical transmission system 20.

Referring to FIG. 41, the optical dispersion compensator unit 32includes three stages of optical dispersion compensator units 25A to 25Dof a plurality of different kinds (four kinds having dispersion amountsB1 to B4 in the arrangement shown in FIG. 41) having differentdispersion amounts having different positive and negative signs, andswitches (switching means) 29A to 29C connected to the three stages ofoptical dispersion compensator units 25A to 25D for switching theselective combination of the optical dispersion compensator units 25A to25D. When each of the switches 29A to 29C is operated for switching, oneof the four kinds of optical dispersion compensator units 25A to 25D ofthe corresponding stage is selected, and consequently, by operation ofthe switches 29A to 29C, a suitable combination of three opticaldispersion compensator units 25A to 25D can be selectively incorporatedinto the optical transmission system 20.

It is to be noted that each of the switches 29A to 29C may be means forwiring any of the optical dispersion compensator advance in each of thetransmitter 21, the repeaters 22 and the receiver 23 of the opticaltransmission system 20 in such a connected condition that thecombination of optical dispersion compensator units 25A to 25D can beselectively switched by way of the switches 29A to 29C, a suitablecombination of optical dispersion compensator units 25A to 25D isselected from within the optical dispersion compensator units 25A to 25Dby operating the switches 29A to 29C. Particularly where theconstruction shown in FIG. 43 is employed, the combination of opticaldispersion compensator units 25A to 25D can be automatically changedover to a combination which provides optimum transmissioncharacteristics to the optical transmission system 20.

It is to be noted that, while, in the embodiment described above,description has been given of the case wherein an optical dispersioncompensator unit 32 is built in each of the transmitter 21, therepeaters 22 and the receiver 23 which constitute the opticaltransmission system 20, advantages similar to those described above canbe obtained where such optical dispersion compensator unit 32 is builtin at least one of the transmitter 21, the repeaters 22 and the receiver23.

The present invention is not limited to the specifically describedembodiment, and variations and modifications may be made withoutdeparting from the scope of the present invention.

What is claimed is:
 1. A method comprising: providing a plurality ofsignal channels associated with different wavelengths, each of theplurality of signal channels arranged to a longer wavelength side of azero dispersion wavelength of an optical fiber transmission line;transmitting, through the optical fiber transmission line, opticalsignals having different wavelengths within awavelength-division-multiplexing signal bandwidth through the pluralityof signal channels; and collectively compensating dispersion given tothe optical signals transmitted through the signal channels.
 2. A methodaccording to claim 1, wherein said collectively compensating dispersioncomprises: dividing the optical signals into a plurality of firstoptical signals and a plurality of second optical signals of whichwavelengths are longer than wavelengths of the first optical signals,and compensating the first optical signals and the second opticalsignals separately.
 3. A method according to claim 1, wherein saidcollectively compensating dispersion comprises: collectivelycompensating dispersion given to all the optical signals transmittedthrough the signal channels.
 4. A method according to claim 1, whereinsaid providing a plurality of signal channels comprises: setting channelspacing between a pair of adjacent signal channels an integer multipleof a minimum channel spacing defined in terms of an optical frequency oran optical wavelength.
 5. A method according to claim 1, wherein saidproviding a plurality of signal channels comprises: providing theplurality of signal channels so that the plurality of signal channelsare equally spaced.
 6. A method according to claim 1, wherein saidproviding a plurality of signal channels comprises: providing theplurality of signal channels so that the plurality of signal channelsare unequally spaced.
 7. A method according to claim 1, furthercomprising: providing a four wave mixing suppressing guard band so as tohave a predetermined bandwidth and include the zero dispersionwavelength, each signal channel being arranged outside the guard band.8. A method comprising: providing a plurality of signal channelsassociated with different wavelengths, each of the plurality of signalchannels arranged to a shorter wavelength side of a zero dispersionwavelength of an optical fiber transmission line; transmitting, throughthe optical fiber transmission line, optical signals having differentwavelengths within a wavelength-division-multiplexing signal bandwidththrough the plurality of signal channels; and collectively compensatingdispersion given to the optical signals transmitted through the signalchannels.
 9. A method according to claim 8, wherein said collectivelycompensating dispersion comprises: dividing the optical signals into aplurality of first optical signals and a plurality of second opticalsignals of which wavelengths are longer than wavelengths of the firstoptical signals, and compensating the first optical signals and thesecond optical signals separately.
 10. A method according to claim 8,wherein said collectively compensating dispersion comprises:collectively compensating dispersion given to all the optical signalstransmitted through the signal channels.
 11. A method according to claim8, wherein said providing a plurality of signal channels comprises:setting channel spacing between a pair of adjacent signal channels aninteger multiple of a minimum channel spacing defined in terms of anoptical frequency or an optical wavelength.
 12. A method according toclaim 8, wherein said providing a plurality of signal channelscomprises: providing the plurality of signal channels so that theplurality of signal channels are equally spaced.
 13. A method accordingto claim 8, wherein said providing a plurality of signal channelscomprises: providing the plurality of signal channels so that theplurality of signal channels are unequally spaced.
 14. A methodaccording to claim 8, further comprising: providing a four wave mixingsuppressing guard band so as to have a predetermined bandwidth andinclude the zero dispersion wavelength, each signal channel beingarranged outside the guard band.
 15. A method comprising: providing aplurality of signal channels associated with different wavelengths,signal channels of the plurality of signal channels arranged to both alonger wavelength side and a shorter wavelength side of a zerodispersion wavelength of an optical fiber transmission line;transmitting, through the optical fiber transmission line, opticalsignals having different wavelengths within awavelength-division-multiplexing signal bandwidth through the pluralityof signal channels; and collectively compensating dispersion given tothe optical signals transmitted through the signal channels.
 16. Amethod according to claim 15, wherein said collectively compensatingdispersion comprises: dividing the optical signals into a plurality offirst optical signals and a plurality of second optical signals of whichwavelengths are longer than wavelengths of the first optical signals,and compensating the first optical signals and the second opticalsignals separately.
 17. A method according to claim 15, wherein saidcollectively compensating dispersion comprises: collectivelycompensating dispersion given to all the optical signals transmittedthrough the signal channels.
 18. A method according to claim 15, whereinsaid providing a plurality of signal channels comprises: setting channelspacing between a pair of adjacent signal channels an integer multipleof a minimum channel spacing defined in terms of an optical frequency oran optical wavelength.
 19. A method according to claim 15, wherein saidproviding a plurality of signal channels comprises: providing theplurality of signal channels so that the signal channels arranged to thelonger wavelength side of the zero dispersion wavelength are equallyspaced.
 20. A method according to claim 15, wherein said providing aplurality of signal channels comprises: providing the plurality ofsignal channels so that the signal channels arranged to the shorterwavelength side of the zero dispersion wavelength are equally spaced.21. A method according to claim 15, wherein said providing a pluralityof signal channels comprises: providing the plurality of signal channelsso that the signal channels arranged to the longer wavelength side ofthe zero dispersion wavelength are unequally spaced.
 22. A methodaccording to claim 15, wherein said providing a plurality of signalchannels comprises: providing the plurality of signal channels so thatthe signal channels arranged to the shorter wavelength side of the zerodispersion wavelength are unequally spaced.
 23. A method according toclaim 15, further comprising: providing a four wave mixing suppressingguard band so as to have a predetermined bandwidth and include the zerodispersion wavelength, each signal channel being arranged outside theguard band.
 24. A method comprising: producing a plurality of opticalsignals having different wavelengths; multiplexing the plurality ofoptical signals into a wavelength division multiplexed signal;transmitting the wavelength division multiplexed signal through anoptical fiber transmission line having a zero dispersion wavelength;collectively compensating dispersion given to the optical signalstransmitted through the optical fiber transmission line; demultiplexingthe wavelength division multiplexed signal from the optical fibertransmission line into the plurality of optical signals; and receiving arespective optical signal of the demultiplexed plurality of opticalsignals, wherein each of the plurality of optical signals is associatedwith one of a plurality of signal channels through which the opticalsignals are transmitted, and each of the plurality of signal channels isarranged to a longer wavelength side of the zero dispersion wavelength.25. A method according to claim 24, wherein said collectivelycompensating dispersion comprises: dividing the optical signals into aplurality of first optical signals and a plurality of second opticalsignals of which wavelengths are longer than wavelengths of the firstoptical signals, and compensating the first optical signals and thesecond optical signals separately.
 26. A method according to claim 24,wherein said collectively compensating dispersion comprises:collectively compensating dispersion given to all the optical signalstransmitted through the signal channels.
 27. A method according to claim24, wherein said providing a plurality of signal channels comprises:setting channel spacing between a pair of adjacent signal channels aninteger multiple of a minimum channel spacing defined in terms of anoptical frequency or an optical wavelength.
 28. A method according toclaim 24, wherein the plurality of signal channels are equally spaced.29. A method according to claim 24, wherein the plurality of signalchannels are unequally spaced.
 30. A method according to claim 24,further comprising: providing a four wave mixing suppressing guard bandso as to have a predetermined bandwidth and include the zero dispersionwavelength, each signal channel being arranged outside the guard band.31. A method comprising: producing a plurality of optical signals havingdifferent wavelengths; multiplexing the plurality of optical signalsinto a wavelength division multiplexed signal transmitted through anoptical fiber transmission line having a zero dispersion wavelength;collectively compensating dispersion given to the optical signalstransmitted through the optical fiber transmission line; demultiplexingthe wavelength division multiplexed signal from the optical fibertransmission line into the plurality of optical signals; and receiving arespective optical signal of the demultiplexed plurality of opticalsignals, wherein each of the plurality of optical signals is associatedwith one of a plurality of signal channels through which the opticalsignals are transmitted, and each of the plurality of signal channels isarranged to a shorter wavelength side of a zero dispersion wavelength ofthe optical fiber transmission line.
 32. A method according to claim 31,wherein said collectively compensating dispersion comprises: dividingthe optical signals into a plurality of first optical signals and aplurality of second optical signals of which wavelengths are longer thanwavelengths of the first optical signals, and compensating the firstoptical signals and the second optical signals separately.
 33. A methodaccording to claim 31, wherein said collectively compensating dispersioncomprises: collectively compensating dispersion given to all the opticalsignals transmitted through the signal channels.
 34. A method accordingto claim 31, wherein said providing a plurality of signal channelscomprises: setting channel spacing between a pair of adjacent signalchannels an integer multiple of a minimum channel spacing defined interms of an optical frequency or an optical wavelength.
 35. A methodaccording to claim 31, wherein the plurality of signal channels areequally spaced.
 36. A method according to claim 31, wherein theplurality of signal channels are unequally spaced.
 37. A methodaccording to claim 31, further comprising: providing a four wave mixingsuppressing guard band so as to have a predetermined bandwidth andinclude the zero dispersion wavelength, each signal channel beingarranged outside the guard band.
 38. A method comprising: producing aplurality of optical signals having different wavelengths; multiplexingthe plurality of optical signals into a wavelength division multiplexedsignal transmitted through an optical fiber transmission line having azero dispersion wavelength; collectively compensating dispersion givento the optical signals transmitted through the optical fibertransmission line; demultiplexing the wavelength division multiplexedsignal from the optical fiber transmission line into the plurality ofoptical signals; and receiving a respective optical signal of thedemultiplexed plurality of optical signals, wherein each of theplurality of optical signals is associated with one of the plurality ofsignal channels through which the optical signals are transmitted, andthe plurality of signal channels are arranged to both a longerwavelength side and shorter wavelength side of the zero dispersionwavelength.
 39. A method according to claim 38, wherein saidcollectively compensating dispersion comprises: dividing the opticalsignals into a plurality of first optical signals and a plurality ofsecond optical signals of which wavelengths are longer than wavelengthsof the first optical signals, and compensating the first optical signalsand the second optical signals separately.
 40. A method according toclaim 38, wherein said collectively compensating dispersion comprises:collectively compensating dispersion given to all the optical signalstransmitted through the signal channels.
 41. A method according to claim38, wherein said providing a plurality of signal channels comprises:setting channel spacing between a pair of adjacent signal channels aninteger multiple of a minimum channel spacing defined in terms of anoptical frequency or an optical wavelength.
 42. A method according toclaim 38, wherein the signal channels arranged to the longer wavelengthside of the zero dispersion wavelength are equally spaced.
 43. A methodaccording to claim 38, wherein the signal channels arranged to theshorter wavelength side of the zero dispersion wavelength are equallyspaced.
 44. A method according to claim 38, wherein the signal channelsarranged to the longer wavelength side of the zero dispersion wavelengthare unequally spaced.
 45. A method according to claim 38, wherein thesignal channels arranged to the shorter wavelength side of the zerodispersion wavelength are unequally spaced.
 46. A method according toclaim 38, further comprising: providing a four wave mixing suppressingguard band so as to have a predetermined bandwidth and include the zerodispersion wavelength of the optical transmission line, each signalchannel being arranged outside the guard band.
 47. A method comprising:providing a plurality of signal channels having different wavelengthswithin a wavelength-division-multiplexing signal bandwidth, each signalchannel associated with one of a plurality of optical signals, eachoptical signal being transmitted through the associated signal channelthrough an optical fiber transmission line; and collectivelycompensating dispersion given to the optical signals transmitted throughthe signal channels, wherein each of the plurality of signal channels isarranged to a longer wavelength side of a zero dispersion wavelength ofthe optical fiber transmission line.
 48. A method according to claim 47,wherein said collectively compensating dispersion comprises: dividingthe optical signals into a plurality of first optical signals and aplurality of second optical signals of which wavelengths are longer thanwavelengths of the first optical signals, and compensating the firstoptical signals and the second optical signals separately.
 49. A methodaccording to claim 47, wherein said collectively compensating dispersioncomprises: collectively compensating dispersion given to all the opticalsignals transmitted through the signal channels.
 50. A method accordingto claim 47, wherein said providing a plurality of signal channelscomprises: setting channel spacing between a pair of adjacent signalchannels an integer multiple of a minimum channel spacing defined interms of an optical frequency or an optical wavelength.
 51. A methodaccording to claim 47, wherein said providing a plurality of signalchannels comprises: providing the plurality of signal channels so thatthe plurality of signal channels are equally spaced.
 52. A methodaccording to claim 47, wherein said providing a plurality of signalchannels comprises: providing the plurality of signal channels so thatthe plurality of signal channels are unequally spaced.
 53. A methodaccording to claim 47, further comprising: providing a four wave mixingsuppressing guard band so as to have a predetermined bandwidth andinclude the zero dispersion wavelength, each signal channel beingarranged outside the guard band.
 54. A method comprising: providing aplurality of signal channels having different wavelengths within awavelength-division-multiplexing signal bandwidth, each signal channelassociated with one of a plurality of optical signals, each opticalsignal being transmitted through the associated signal channel throughan optical fiber transmission line; and collectively compensatingdispersion given to the optical signals transmitted through the signalchannels, wherein each of the plurality of signal channels is arrangedto a shorter wavelength side of a zero dispersion wavelength of theoptical fiber transmission line.
 55. A method according to claim 54,wherein said collectively compensating dispersion comprises: dividingthe optical signals into a plurality of first optical signals and aplurality of second optical signals of which wavelengths are longer thanwavelengths of the first optical signals, and compensating the firstoptical signals and the second optical signals separately.
 56. A methodaccording to claim 54, wherein said collectively compensating dispersioncomprises: collectively compensating dispersion given to all the opticalsignals transmitted through the signal channels.
 57. A method accordingto claim 54, wherein said providing a plurality of signal channelscomprises: setting channel spacing between a pair of adjacent signalchannels an integer multiple of a minimum channel spacing defined interms of an optical frequency or an optical wavelength.
 58. A methodaccording to claim 54, wherein said providing a plurality of signalchannels comprises: providing the plurality of signal channels so thatthe plurality of signal channels are equally spaced.
 59. A methodaccording to claim 54, wherein said providing a plurality of signalchannels comprises: providing the plurality of signal channels so thatthe plurality of signal channels are unequally spaced.
 60. A methodaccording to claim 54, further comprising: providing a four wave mixingsuppressing guard band so as to have a predetermined bandwidth andinclude the zero dispersion wavelength, each signal channel beingarranged outside the guard band.
 61. A method comprising: providing aplurality of signal channels having different wavelengths within awavelength-division-multiplexing signal bandwidth, each signal channelassociated with one of a plurality of optical signals, each opticalsignal being transmitted through the associated signal channel throughan optical fiber transmission line; and collectively compensatingdispersion given to the optical signals transmitted through the signalchannels, wherein signal channels of the plurality of signal channelsare arranged to both a longer wavelength side and a shorter wavelengthside of a zero dispersion wavelength of the optical fiber transmissionline.
 62. A method according to claim 61, wherein said collectivelycompensating dispersion comprises: dividing the optical signals into aplurality of first optical signals and a plurality of second opticalsignals of which wavelengths are longer than wavelengths of the firstoptical signals, and compensating the first optical signals and thesecond optical signals separately.
 63. A method according to claim 61,wherein said collectively compensating dispersion comprises:collectively compensating dispersion given to all the optical signalstransmitted through the signal channels.
 64. A method according to claim61, wherein said providing a plurality of signal channels comprises:setting channel spacing between a pair of adjacent signal channels aninteger multiple of a minimum channel spacing defined in terms of anoptical frequency or an optical wavelength.
 65. A method according toclaim 61, wherein said providing a plurality of signal channelscomprises: providing the plurality of signal channels so that theplurality of signal channels are equally spaced.
 66. A method accordingto claim 61, wherein said providing a plurality of signal channelscomprises: providing the plurality of signal channels so that theplurality of signal channels are unequally spaced.
 67. A methodaccording to claim 61, further comprising: providing a four wave mixingsuppressing guard band so as to have a predetermined bandwidth andinclude the zero dispersion wavelength, each signal channel beingarranged outside the guard band.